Alone is best but sometimes you need mates. Worse comes to worst, you can always eat em.
Slark, the Nightcrawler
Lore:"Little known to the inhabitants of the dry world, Dark Reef is a sunken prison where the worst of the sea-breed are sent for crimes against their fellows. It is a razor barbed warren full of murderous slithereen, treacherous Deep Ones, sociopathic meranths. In this dim labyrinth, patrolled by eels and guarded by enormous anemones, only the vicious survive. Pitched into Dark Reef for crimes unknown, Slark spent half a lifetime without kin or kindness, trusting no one, surviving through a combination of stealth and ruthlessness, keeping his thoughts and his plans to himself. When the infamous Dark Reef Dozen plotted their ill-fated breakout, they kept their plans a perfect secret, murdering anyone who could have put the pieces together--but somehow Slark discovered their scheme and made a place for himself in it. Ten of the Dozen died in the escape attempt, and two were captured, hauled back to Dark Reef, then executed for the entertainment of their fellow inmates. But Slark, the unsung thirteenth, used the commotion as cover and slipped away, never to be caught. Now a furtive resident of the carnivorous mangrove scrub that grips the southern reach of Shadeshore, Slark remains the only successful escapee from Dark Reef."
Slark is an agility escape melee hero with a lot of carry potential if you play him right. I personally did it very wrong in the beginning, wound up getting focused on and never got even close to an Eye of Skadi. I changed the way I played and suddenly end up with a few thousand gold before I even know what I want to buy next.
My win percentage with Slark is about 60%, some 10% higher than the average. Even when we are losing badly I end up doing very well with Slark, but we still lose thanks to feeding of other heroes on my team. I tend to believe I know what I'm doing. UPDATE May 15th 2013: I added a second build! I've been experimenting a lot after various comments and also after watching the TI3 West Qualifiers. I find this build equally effective as my previous one, but it's just slightly different.
First of all, Slark is a very squishy hero, and if you get focused early on in fights you will die very fast. This is the reason I go with Stout Shield. You could get Poor Man's Shield if you'd like. I personally don't because I get rid of the shield later on anyway. So building it into a Vanguard is also pointless. If you find yourself needing health regen in a teamfight it's not something Vanguard can fix and you should use Shadow Dance instead.
Let's go over his stats and the possibilities Strength: 21 + 1.8 Agility: 15 + 1.5 Intelligence: 16 + 1.9
Even though he is an agility hero, he gains intelligence the most per level. Ring of Aquila and Magic Wand buff this a little bit early game, and later on you'll get a lot of stat buffs thanks to Eye of Skadi.
His movement speed is 305. This goes up to 360 when you get Power Treads and once you get his ult you get a 30%/35%/40% bonus movement speed when you're not visible to the enemy team. You are able to hit max movement speed (522) with Boots of Speed and Sange when you are invisble.
He starts with 1.1 armor. But since he is an agility hero and you'll be boosting your stats anyway armor isn't really a problem. Of course, since you're getting Ring of Aquila you'll a bonus from that as well.
The Power Treads can be used for extra health, damage or mana depending on your style. I personally keep it on agility for the damage. If you're still laning without Shadow Dance I advice to keep it on strength. Also: please buy Belt of Strength first. The extra health early game is much more important than the attack speed you get from Gloves of Haste!
Depending on how well you're doing you're either still in lane while getting the gold for Ring of Aquila or you're rotating lanes to push/defend towers. I personally have a lot of problems with mana. That's the main reason I get Ring of Aquila, but also for some extra boost in stats and armor. I usually end up with 1000 gold before I know it so I can buy the ring in one piece. If you don't have the luxury or die with having around 500 gold I'd advice to get the Ring of Basilius first.
Remember to turn off the Ring when you don't want to push a lane and to keep it on when you do. It gives creeps a bit of extra armor making them a bit tougher to kill.
Other items for mana could be Soul Ring or Arcane Boots, but I'd recommend against both. You're too squishy for the Soul Ring and it doesn't weigh up against the stats and armor from Ring of Aquila. I wouldn't completely advice against Arcane Boots, since the mana is very handy, but the cost is rather high. With a bit of luck a teammate will get it instead and you can request the mana from them.
Next is when I go for Shadow Blade. This works great with Shadow Dance, because the blade gives you 9 seconds of invisibility. Say you're level 16 and you have levelled your Shadow Dance three times. That means 9 x 7% = 63% of your health back, while you're scouting the map for enemies at the same time. Also, when you're starting to get in the kills they are more likely to run from you on sight. Shadow Blade prevents this. Do be cautious though, getting a Shadow Blade increases the odds of the enemy team buying true sight (wards, gem or dust). Get Claymore first, you don't have much use for the Shadow Amulet.
Cost: 10625 gold
I use Radiance to up my own damage, which can be relatively low if you haven't hit an enemy in the last two minutes. And eventually for the burn damage. It helps you clear out creepwaves even faster in combination with your Dark Pact. Also when you use Pounce, the enemy hero has no choice but to take the burn damage. Do remember to turn of the burn damage if you're using Shadow Blade to scout and don't want to be noticed. Late game I neglect doing this because the burn damage is just another reason for them to run. Radiance replaces Stout Shield as your inventory is rather full at this point. Eye of Skadi can be used to keep any fleeing heroes at bay. Combined with Pounce this should secure most kills. If an enemy still seems to be faster than you (because of a haste rune, Surge or items like Force Staff remember you should have your Shadow Blade to catch up.
Usually when I get Eye of Skadi the match is practically over. But if not you could get some items from the luxury list to replace items you have. I'd say that you should replace your Magic Wand with whatever you're getting. After that it's whatever you feel is least effective.
Of course a good player always carries a Town Portal Scroll. Unfortunately the way I build Slark doesn't leave much room open for this. His speed usually makes up for any delayed entry and if your map awareness is good enough you will see any tower push coming and be ready for it. If you're not, make sure you adjust the build so that you can carry the scrolls.
The first skill you want to level is Pounce. This is your escape as well as your initiation. If you use it right, you can dodge most attacks. This also includes abilities: I've dodged stuff like Sniper's Assassinate and Bounty Hunter's Shuriken Toss, which is very handy if you're low health. This requires some timing and I honostly don't know if this is supposed to happen!
Ability: No Target (Area of Effect) Affects: Enemies (and Slark) Damage: Magical
Can be blocked with Black King Bar
Radius: 325 Delay: 1.5 Total damage: 75/150/225/300 (damage to Slark: 37.5/75/112.5/150)
Mana cost: 55/50/45/40
After a short delay, Slark sacrifices some of his life blood, purging negative buffs and dealing damage to enemy units around him and to himself. Slark only takes 50% of the damage.
The first skill you want to max is Dark Pact. This serves as an excellent farming tool if you're alone in lane. At level 4 you walk around the melee creeps, hit the ranged creep(s) once and use dark pact. You can then finish off the rest of the melee creeps, stay out of vision to heal up and move on to the next wave.
At low level this is simply a nuke to enemies, who have very low health. It does about 225 damage to enemy heroes on level 4, and about 112 damage to yourself. Remember that it's a Area of Effect spell, so you can still damage invisible heroes if you know where they are.
The damage comes in pulses, meaning if someone moves away from you while the pulses are emiting they can 'dodge' some of the damage. Every pulse removes debuffs. The pulses are 1 second in total (bringing the total duration of this skill to 2.5 seconds).
Ability: No Target
Affects: Enemy Heroes (one)
Can be blocked with Black King Bar
Slark leaps forward, grabbing the first hero he connects with. That unit takes damage and is leashed to Slark, and can only move a limited distance away from Slark's landing position.
The second skill you max is Pounce. This just updates the damage you do, but also decreases your cooldown to 8. This means you can pounce into a teamfight (on a squishy hero who's kiting perhaps!) en pounce out later (or pounce another hero!).
You will pounce in the direction you are facing, so make sure you are looking directly at the hero you want to pounce (or look at the point he's going to be).
The duration of the leash is not long enough to use Dark Pact twice. The radius is small enough to safely use it without the risk of the enemy not being inside the range.
The leash radius is larger if enemy heroes use a haste rune, or are otherwise extremely fast (thansk to Dark Seer's Surge for example.
You leap with a speed of 933.33, and you can jump over obstacles such as trees. If you land inside trees they will be destroyed in a 200200 area around you.
NOTE: when you are fighting a hero that can go invisible, try to pay attention to the leash. This works very well when you're fighting Phantom Lancer for example. Even though he can go invisible and you see an illusion run away, you can still see the leash being in effect. As long as that's there, you know he's still there (if you used Pounce on the right hero to begin with of course!
Slark steals the life essence of enemy heroes with his attacks, draining each of their attributes and converting them to bonus Agility.
And lastly, besides speccing it once at level 4, the last skill you get is Essence Shift. Early on the 15 seconds is fine for teamfights. Later on when fights occur more often or when you're picking them off one by one the two minutes is very handy to get up your damage.
Ability: No Target Affects: Self Duration: 5.5 Bonus Movement Speed: 30%/35%/40% Health Gained per Second: 3%/5%/7% Cooldown: 25 Mana Cost: 120
When used, Slark hides himself in a cloud of shadows, becoming invisible (attacking, casting spells, and using items do not reveal Slark). Passively, when not visible to the enemy team, Slark gains bonus movement speed and health regeneration.
Get your ult, Shadow Dance, every time you can. The health regen works miracles and the speed gives you nice mobility. Try not to use it in teamfights when it's not necessary, unless you're getting disabled. If you 1v1 a disabler you definitely use it. Also good to use when low on health. This is a good skill to chase down enemies who try to get away or as escape. Remember you can use skills, items and attack when invisble in the cloud. This includes using a Teleport Scroll.
Keep an eye out for the auras that have effect on you. If you're walking around alone and your ult is not one of them, there is vision on you. This works well to scout wards. Do know that it's bugged sometimes (like when you don't have the aura from it when you're at your fountain). When you attack neutral creeps the passive effect of this skill will be disabled for 2 seconds. And enemies don't have to actually watch you to 'see' you, as long as you're visible to them on the minimap your passive does not work.
When laning, try to get into a lane with a ranged support who can help you get farm. Just follow the normal rules to get to level 6 asap (don't take risks, don't leave lane too much, etc.)
I usually start killing tons of people as soon as I have Shadow Blade. Just find someone who is alone, and start attacking them from invisibility. If it's a disabler consider using your ult, if it's not just keep auto attacking. Save Pounce for when they try to get away. You can use Dark Pact straight off, just don't abuse it. If they go invisible on low health you need to be able to Pounce and use Dark Pact ( Riki, Bounty Hunter and Templar Assassin won't be grateful when you do).
Your enemies come in other forms. Gem of True Sight, Lvl 3 Necronomicon and Sentry wards are the way to counter Slark. Especially when you use Shadow Dance which lets enemies know you are near, odds are you will get revealed with Dust. Which is another reason I use Shadow Blade. Just Purge whatever you can and go invisible!
When do you buy the situational itemas I have listed above? As you've seen I've put some items up as rejected, here I'll tell you why. With rejected I mean I usually don't consider purchasing them, but they can be usefull in some situations. Sorted alphabetically:
Aegis of the Immortal - Obiously this isn't much of a purchase. You can solo Roshan late-game, but I usually prefer to do it with teammates. I also give the Aegis away (since Slark has 3 escapes with my build), but if you feel you are dying to early in teamfights and this makes your team lose the fight you should get it yourself so you can wreak havoc after 'dying' the first time and push the hell out of the lanes when the enemy dies. This also allows you to play a bit more recklessly.
Armlet of Mordiggian - This is actually a great addition on Slark. The extra strength makes you less squishy and when activated you get a great additional attack speed.
Battlefury - If your team has a the oppertunity to get enemy heroes together (like Black Hole or Vaccuum this will be a great addition to Slark. It also gives you some (much needed) mana regen and of course a bit extra (unneeded) health regen). Obviously the extra damage is nice too.
Butterfly - Evasion usually works best if you already have a lot of HP, but it also works fine for Slark. It also gives you nice damage.
Heart of Tarrasque - Shadow Dance is basically a free heart. BETTER than a free heart. If you want to expand the effect (because it stacks) feel free to get your 9% health regen). I personally don't really see the use of it, I never have health problems myself.
Mask of Madness - While the increased attackspeed synergises great with your Essence Shift the increased damage is terrible. Slark if very squishy and the increased damage can kill you very fast. If you find yourself revealed a lot (by Dust, Gem or Wards) DO NOT get Mask of Madness. If the other team does not take actions against your invisibility this is a decent option.
Vanguard - The damage block and regen is nice early game (which is why I DO get Stout Shield, but you don't need it late-game. Again if you get revealed a lot the block would be nice but I find it does very little to prevent your death once you're revealed.