(Imaged credited to CyborgMatt, taken from his blog)
Nyx Assassin, formerly Nerubian Assassin, is an early game disabler, mid game nuker with semi-carry potential. Unfortunately on this site the third ability, "Urna Swarm" is in the place of the ability that should be listed, "Spiked Carapace". So when reading the skill sequence, keep in mind that "Urna Swarm" is actually "Spiked Carapace". Nyx, or Anub'Arak for veterans, is a disabler/nuker who eats squishy carries or supports. When playing him, keep in mind that you aren't a carry and cannot take on 3 people, no matter how fed you are. This guide focuses a lot on items, and although a lot of heroes are flexible in this respect, buy what in certain conditions against certain foes with certain teammates is important. Remember that the item build order I suggest should not be followed religiously, as DoTA is an amorphous game and there are many items to be built in different situations.
Impale - Similar to Lion's, the only differences being that Lion's mana cost is higher and Nyx's stun is .77 seconds longer.
Mana Burn - Takes away mana equal to four times the target's intelligence and damages them for the same amount. This means it is generally more effective to intelligence heroes, especially Outworld Destroyer, who stacks intelligence more than most.
Spiked Carapace - This will reflect the next instance of damage to whoever hit you. If you are hit by a stun while this is after, it will still stun you, but the damage will be negated and reflected. This does not work against DoT's similar to Axe's Battle Hunger. Beware of Juggernaut's spin, it will eat your carapace without any effect on Juggernaut. Note that this does not reflect spells as a whole, only damage dealt to you, meaning that if Skeleton King uses Hellfire Blast on you, you will reflect the initial damage, but still be stunned, take the remaining damage over time, and be slowed.
Vendetta - This will turn you invisible, make you go faster, and you will deal a large amount of bonus damage only if you break it with a base attack. The invis has no delay, so you can use it to dodge spells if you use it fast enough. Another cool bonus is that this gives you a unit walking effect, like Phase Boots. You will be able to run through a creep wave to snag a kill.
-A full combo (with Dagon) can kill
pretty much any agility/intelligence hero
-Very high base HP regen (3.2 or something)
which allows you to shrug off the occasional
-Screws over intelligence mid heroes like
Storm Spirit or Outworld Destroyer
-Not item dependant after you get/upgrade
Dagon with this build
-Silences make you pretty useless
-Kind of weak until level 6
-Dependant on Dagon with this (Nuker) build
-Weak versus strength heroes (especially
Centaur and Naix)
-Getting free kills may get boring
-Sentry wards make your ganks a lot harder
Nyx Assassin is notorious for his mana burn that scales indefinitely. If the enemy team has 3 or more intelligence heroes, you have an excuse to choose NA. If they have Outworld Destroyer, choose NA. Na is Outworld Destroyer's hard counter, the Mana Burn, Spiked Carapace, physical damage nuke, and stun make for a good counter to Outworld Destroyer. The reason the Mana Burn is so effective against OD is that it scales off of 4x the target's intelligence. OD stacks intelligence more than most because his ultimate scales off of it. Hitting OD for 1000+ with Mana Burn is not unheard of.
If you see that their team has a lot of squishy intelligence heroes or agility carries, picking Nyx is also a good idea.
Nyx Assassin is not a carry, nor a support. He is a disabler, nuker, and ganker. You generally won't want to go mid lane with this build (I suggest 3 branches and a tango, to save up for a bottle if you do go mid lane), try to avoid solo side laning or tri-laning as well. You lane well with other gankers or even carries who can nuke early game(Gondar, Morphling, Huskar, Luna, Juggernaut, Ursa, and maybe even Viper). Even though my guide calls for buying Dagon, in no way does that mean I want you to run around taking all the kills with it, if you're with a carry and you're about to kill someone, let the carry take it, don't Dagon-steal it. Early game, all you do is set up kills and ganks, mid game, you're a one-man-ganking-machine who roams around eating squishy intelligence heroes along with farming carries. Don't buy carry items like Butterfly, Daedalus, and Satanic, buy items that add to your CC arsenal or assist in your nuking. Health and mana are a priority for you, to be able to weave in and out of team fights is important, and spamming your Mana Burn and Spiked Carapace is also important. In team fights, you are less of a nuker and more of a disabler. Hitting 3 or more people with your Impale is critical, and getting a carry to target you so you can reflect their 500 damage crit back at them also does a service for your team.
Starting Items - I chose the gauntlet and mantle for the extra HP and mana, because even though NA is an agility hero, he doesn't really benefit from stacking it as much as you'd think. The tango and salve is a common choice, but you can trade out the branches for more tango or another salve, if you want.
~12 Minutes - Arcane boots are absolutely great on NA. He isn't intelligence based and thus has mana problems. The bracer is a cheap ways= to gain stats, notably HP (1 bracers gives you 114 HP). The Null Talisman is there to be built into a Dagon, as well as the latent stats.
~25 Minutes - A level 1 Dagon should definitely be acquired before 25 minutes, but we've all had those games where you are gold depraved. Dagon is to be used in conjunction with your Vendetta to one-man gank any over-extended squishies, such as a glass cannon agility carry or a Furion getting cocky with the pushes. A good combo would be to sneak up on them with Vendetta, hit them for the high bonus damage, Impale, ready your Spiked Carapace, Dagon, and if they are too far to auto attack and have just a wee bit of HP, mana burn them to finish them off. I recommend leveling Dagon to 5 before building any luxury items. Drum of Endurance is just one of those items that are good to pick up. This should be bought before Dagon, but if you're finding it hard to get any decent assassinations off, go ahead and skip it.
Luxury - Assuming you leveled Dagon to level 5, you should do enough burst damage to nuke someone pretty low, if not dead. It's time to build one of those shiny luxury items! If you feel like your game is going to go on for a long time, and you're going to need to be in many places at once, grab Boots of Travel first. These are amazing late game and are needed for sudden pushes and team fight presence. Heart of Tarrasque is always a good choice if you pissed off the enemy team enough to make them want to focus you every team fight, as it gives you a whopping 1060 HP(1 strength point gives you 19 hp, HoT has 40 strength and 300 flat bonus HP, 40 x 19=760, 760+300=1060 hp)! Ethereal Blade is always a good choice on NA, simply because it's active effect adds some damage to your combo(75 flat damage+2x your agility) AND makes the enemy takes %40 bonus magic damage + a %60 slow for 3 seconds, making your Dagon, Impale, and Mana Burn hit harder for 3 seconds(Beware, while this 3 second debuff is on them, they are immune to base attacks, that includes Vendetta)! I like to build Ethereal Blade over Refresher Orb, as a full Refresher Orb rotation will leave you with no mana and you can only perform this rotation every 160 seconds(2 minutes and 40 seconds), and Ethereal Blade adds another solid damaging active to your repeteur. Eul's Scepter of Divinity is good if you decided to be your team's ward buyer and you decide that your role is to be a disabler, not a nuker. The bonus speed is nice, but the magic is in the active effect, Cyclone. Cyclone does not do any damage, but prevents the unit you cast it on from taking or dealing any sort of damage for 2.5 seconds, they can not move while effected by this. You can cast this on allies as well as your self, but beware, use it sparingly, you may have made the wrong enemy invulnerable or the wrong ally unable to attack.
These are items I have seen in other guides as well as items I have seen people build/try to build in real games.
Skull Basher - You are not an auto attacker with this build, you shouldn't buy this.
Lothar's Edge/ Shadow Blade - People reason that you can fake a Vendetta with this item. Well maybe in Warcraft 3 where the attack animation for Vendetta was the same as an auto attack, but even then it was a waste of 3300(!) gold.
Daedalus - Sure, you have a have a chance to do %250 plus 525 of you auto attack damage every 50 seconds, but it is not reliable and costs nearly 6000(5750) gold. A level 5 Dagon does 800 damage every 20(!) seconds. Also, you aren't a carry and should not be buying items like this. [UPDATE: It is even less useful now that the bonus damage from Vendetta is factored in after the crit; I.E. it was (Damage+Vendetta Bonus)(Crit Multiplier), now it is (Damage)(Crit Multiplier)+Vendetta Bonus].
Heaven's Halberd - I don't really know the reason for buying this on NA, evasion and HP are nice and all, and the active is good for shutting down a fed carry, but why buy it on NA? Think of NA as having the same role as Lina(mid Lina, not ward b*%3@# Lina), rather than Balanar or Sven, two other gankers.
Divine Rapier - I've seen this on 3 guides and I wonder when someone decided that grabbing +300 bonus damage on someone that doesn't auto attack often is a good idea, not to mention that it drops when you die. If you think Vendetta is gonna save you every time, you're wrong, eventually someone is going to buy dust or a gem or wards, and that Divine Rapier that does a whole lot of nothing for you is given to the enemy team. Now their carry who actually uses auto attacks has it.
Eye of Skadi - I've seen atleast 2 other NA guides that mention this(not on this site), as well as many people do well with it in game, but I can't make sense of why. I've tried it and the stat boost was nice, but I didn't find myself utilizing the slow effect too much.
Bloodstone - The Mana and HP boosts are nice, but the mana regeneration is overkill and you won't ever really need that much regen. I think it's a wasted item slot, but I've seen many good players buy it.
Blademail - The reasoning behind people buying this is because you have potential to return %200 of their damage. I don't believe that is true, though, because in the tooltip, it says "Returns any damage you take" With spiked carapace, though, you won't take any damage for 1 hit, thus making Blademail useless for 1 instance of damage.
Black King Bar - You generally shouldn't be getting focused with disables because you really aren't too much of a pest in team fights, but again, I've seen a lot of good players buy this and they were utilizing it well.
Rod of Atos - 25 Intelligence(325 mana and 1 mana regenerated every second) and 250 HP plus a decent slow active effect. But the cost is 3100 gold. Personally, I wouldn't buy it, if I were going to stack intelligence and health and was seeking a nice active effect, I'd buy Necronomicon.
Necronomicon - A great item that summons 2 powerful mana burning creeps. One melee and one ranged creep are produced and they get much more powerful the more the item is upgraded(up to level 3). At level 3, the summoned creeps give true sight and the melee creep has an on death effect that deals 500 damage to whoever killed it. I don't grab it too often because having too many active effects gets confusing and it really only is worth it at rank 3. On top of that the micro-management that comes with summoning these creeps is tough to deal with because they have different functions than eachother/your hero. Having a Rasta on your team is a good remedy to the whole micro-management issue, as he can simply snare them while you and your 2 creeps wail on them.
Guinsoo's Scythe of Vyse - Also known as Sheep-stick or Guinsoo's, this item has a decent attribute boost and nice mana regeneration, but the active effect is what is really nice about this item. Hex turns the target into a cuddly critter who can't do anything but move slowly for 3.5 seconds. They retain all their stats such as armor and health in this state. Another note is that while hexed, the target does not gain experience. This is good in conjuncture with Lion, the chain Hex can really shut someone down.
Shiva's Guard - A tanky item with a good active effect. It gives you 30 intelligence(390 mana and 1.2 mana regeneration) as well as 15 armor. The slow has a large radius and can prevent an entire team from running away after a nice Tidehunter, Earthshaker, or Sand King initiation.
Lion - Lion's nuke and CC synergizes well with your own, although you both have very similar roles, his leaning more towards support and yours leaning more towards semi-carry/disabler. You won't see him and you on a team too often because of this.
Sand King - Sand King and you, despite both being mana dependent melee disablers, are great lane partners, and also synergize in team fights and ganks. Your impale has a longer range than his Burrowstrike, so setting him up to Burrowstrike on them by Impaling first is nice, this will also keep them stun locked.Be careful, though, with you and Sand King on the same team, they enemy team will most likely buy a Gem of True Sight or Dust of Appearance.
Bounty Hunter - Gondar(AKA Bounty Hunter) is a fun person to lane with. If you ever lane with him, act like a lane support. Set up kills, deny creeps, let him take last hits. Later in the game, he can Track people for you to chase down with Vendetta. Track lowers their armor and gives you a speed boost while chasing them. Track also gives bonus gold, when a Tracked hero dies, to you, and Gondar. Be careful, though, with you and Gondar on the same team, they enemy team will most likely buy a Gem of True Sight or Dust of Appearance.
Any Carry With Early Game Burst - Because you aren't a support, yet often end up laning with a carry, your role is often setting up kills for you lane partner. Laning with a carry with early game burst such as Luna, Gondar, Ursa, or Juggernaut, your can Impale and hope your lane partner has enough nuke to take them down with your help.
Enigma - I recently played a game where someone picked Engima, then someone picked Furion immediately after that. Engima, rather than jungling, ended up laning with me. We dominated, I didn't die at all that game, and there was not a whole lot of resisting us when we decided we wanted a kill. Enigma's periodic damage/stun is really good when they get cocky and try to initiate on you. Your Spiked Carapace + Impale combo can keep someone from running for only so long, but the follow up with an Enigma is scary.
Crystal Maiden - Crystal Maiden should usually tri-lane, roam, or just babysit a carry rather than lane with Nyx, but if possible, this is probably one of the best kill lanes with NA. Frost Bite + Impale + Crystal Nova, and, if needed, Mana Burn, followed by some greedy auto attacks/chasing if they aren't dead yet. Again, don't expect/ask a CM to lane with you when you have a carry who needs her, though.
Silencer - This hero can shut down an entire team early game, an AoE and a global silence is devastating and he has a mana burn mechanic as well. Beware him especially.
Huskar - Don't ever try to gank Huskar as NA. He has high base HP and strength growth, making him hard to nuke, and seeing as his damage and attack speed raise incredulously depending how low his health is, when he survives your nuke, you are in for a lot of pain.
Pudge - Pudge has high HP growth and passive magic resistance. Beware of the hooks!
Antimage - A mana burn on his basic attacks, blink, and magic resistance gain make him a scary foe. Because he is agility based, he also has high base agility and agility growth, giving him decent armor to make your Vendetta less effective.
Anyone With a Silence - Nyx relies on abilities and items to get the job done, being silenced makes you pretty much useless for the duration of the silence.
I added a section for items suggested in the comments, please keep commenting (constructive criticism only please), and if you down vote, tell me why, there may be something I missed or need to edit! Thank you ERMAC__ for suggesting Urn of Shadows, that should have been somewhere in there from the beginning, and thank you hades4u for suggesting Force Staff, although I'll have to explain when to buy it and why at some time or another.
Urn of Shadows - Urn of Shadows is a nice item that gives you some HP (6 strength=114 hp) and 50% mana regeneration. The active is really nice, every time an enemy dies near you, you get a stack(unless a teammate who also has Urn of Shadows is closer). A stack can either be used to heal an ally for 400 over 8 seconds, or damage and enemy for 150 over 8 seconds. Whats pretty neat about Urn of Shadows is that only getting hit by a hero interrupts the heal, so jungling or being hit by lane creeps will not interrupt the heal. Urn is a great item to buy on NA, but if someone else wants to buy it (Night Stalker, Witch Doctor, and others who buy it regularly), let them instead of you, splitting Urn Stacks isn't too fun.
Force Staff - Force Staff is a tasteful reference to Star Wars, why not buy it? Well, often times it will do less than it is worth on NA. The only stat you will most likely end up using on Force Staff is the intelligence, the attack speed and bonus damage won't help you a whole lot. Force Staff's active will push whoever is targeted a fair distance in whichever direction they are facing. This can be used to go over cliffs and give you a boost when escaping ganks. You should note that while being pushed, you will destroy any trees you come in contact with. Be careful though, as you can get yourself stuck on plateaus like the wards spot infront of Roshan on the Radiant side. If you're getting trapped by an Earthshaker or similar, Force Staff is a good choice. If they have a Pudge, Force Staff is also a good choice. Using Force Staff on a teammate who is being Dismembered by Pudge will free them from him.
Blink Dagger - Buying Blink Dagger can turn you into an initiator with Impale follow by a Vendetta, which is handy if your team either didn't pick an initiator, or your initiator is making mistakes/is unreliable.
Orchid Malevolence - This is usefull if someone on your team for whatever reason already picked up a Dagon or could be bought in place of Ethereal Blade. The silence on the active is really good if you're trying to gank an Anti-Mage or perhaps a Slark, who can escape pretty much anything easy if they aren't disabled immediately. The second part of the active can finish someone off who just barely gets away.
Diffusal Blade - If bought Orchid Malevolence already, or you think that both Ethereal Blade and as an alternative Orchid Malevolence will not help you, then Diffusal Blade is a good item to pick up. The stats are nice and the mana burn that synergizes with your own Mana Burn ability is also nice, but the active ability is amazing. The active, Purge, slows an enemy and removes any buffs/debuffs. This can also be used on yourself and allies, removing debuffs but also slowing you, making it useful in escapes where you are debuffed or a timed stun is about to go off on you, but also good in chasing after your combo.