For start let me clarify one thing. Necrophos, although he is not a right-clicker, is a carry and core hero. Durable team-fighter of course; but still the hero who needs lots of farm to be really effective and he has potency to get the farm quite quickly. From time to time I still play with players who think Necrophos is a support and even have reported me for not buying wards or courier. Needles to say we often won that game and Necrophos had highest or second highest farm, KDA and level.
I have played about 70 games with Necrophos with a 70% win rate. I tried to analyse lots of competitive matches. At dotabuff.com Necrophos has still the best or second the best win-rate of 58-59%. At datdota.com he maintains a solid 56-57% win-rate and number of picks comparable to Medusa or Venomancer.
As always there is no strict order of picking skills and buying items. You always must adapt to a current situation. But there are some valuable recommendations and standard order of things which can help you to proceed toward victory.
What kind of carry Necrophos is and how to adapt to the current situation are guestions this guide will try to answer.
Death Pulse is one of your two active skills. It's very spammable but not at the beginning of game because of higher manacost. Important thing to realize is it heals you and your allies and deals damage to enemies so effectively it creates a double difference in HPs! Thanks to this mechanic it is well suited for first blood and early kills so try to save your mana for such an early opportunities.
Later in game Death Pulse works as your main farming tools. This is also the first reason why you are a not pure support. With this skill you can farm very effectively and it would be waste to not use it for farming. Try to practice farming lane creeps. It takes two pulses to kill melee creeps and one pulse and one right click to kill ranged creep.
In team fights you are supposed to spam Death Pulse one after another. Over 30 seconds you are able to cast it seven times which translates into 910 HP healed and 1925 damage before magic resistance reduction. Because of this damage/heal mechanic it can create a horrific difference of HPs so Necrophos is also very important team fighter.
You always want to maximize this skill as a first one by level 7.
Heartstopper Aura is quite strong aura I you think about it: first level of this aura can deal 36% of HP per one minute just by standing near of your enemies! Also notice its high range. Because of this it is very often to see people put first point at first level to Heartstopper Aura. Then you can harras your opponents without actually right click them. There is also one important psychological effect similar to micro-payment in economics: People don't realize they are actually loosing so much health. Use this synergy with Death Pulse to score some First Blood and other early kills.
Thanks to this aura you will have a lot of assist kills. So this skill also helps you farm some more gold. Particularly funny is following nearly death hero to get the kill without single click.
Beside it is strong you probably want to maximize this skill as the last one by level 14. By that time you are dealing 45% of max HP over 30 seconds! This is second reason why you are the good team fighter. Also it's a reason why you need durability items to boost your tankiness and let the aura do most of the damage.
Sadist is your second passive skill and second reason why Necrophos is best suited to be a carry and not support. You need those last hits and kills to be effective! But this skill alone does not grant you to get them so it's often better to put points into Death Pulse first. One value point before level 6 is often enough for the early game.
You probably want to max Sadist as a second skill by level 10. Heartstopper Aura is good but you need to stay alive on the battlefield long enough to make that aura effective and Sadist can help you with that. Just beware of Ancient Apparition's Ice Blast. When you are Frostbitten this skill's health regeneration does not work at all and you will die in team fight very quickly.
Reaper's Scythe is signature skill of Necrophos. Very strong but a little ambivalent because you sometimes need to explain to your team pub players you are not stealing their kills (If you dont know: the kill is always accredited to Necrophos even if other hero kills the stunned victim - it synergized with your Heartstopper Aura as well). This may be true in old version of Dota 2 but with recent upgrades which prolongs respawn timer and the buyback disable it's very important to kill enemy heroes with scythe. Also this ultimate has acceptable cooldown of 70 seconds when maxed or with Aghanim's Scepter so don't be afraid to use it often. (See, this is also third reason why you are more the carry than the support)
At first level it does not deal much damage but still it does 1.5 second of stun which goes through spell immunity. Use this for gangs and secure kills in midgame. But at higher level this skill aspires to the best nuke in the game. Let's look at some numbers: First level kill threshold is 23 % of max HP. Second level is 31 % and third level is 40 %. With Aghanim's Scepter it is even 47 % of max HP! And now imagine a Pudge with 6k health.
At competitive gaming the extra 30 seconds of respawn time and Aghanim's Scepter's disable of buyback is often the most important thing why teams pick Necrophos.
Items are much more interesting and more diverse then skills. Also there is a larger gap between what pub players buy and what competitive players buy.
This starting set of items is probably invented and popularized by Eternal Envy or Funn1k. Why two of Ring of Protection you ask? Rings are very cost effective armour and therefore protects you against harrasing right clicks from enemies. They are also build later into Ring of Basilius and Tranquil Boots just from Side Lane Shop. Iron Branch are classic starting items which you can sell later or buy a Magic Wand. One Tango is often enough when you are building Tranquil Boots.
Let me do a short comparison of available boots now before we proceed to early game items.
Those are the worst boots for Necrophos by my guess. You don't need right click damage and similar speed is granted by Tranquil Boots.
Mana boots can be occasionally good when you are forced to do some supporting for you mana hungry friends and can be later disassembled for building Bloodstone.
Very common choice on Necrophos. But I can't get rid of feeling that Power Treads are more suited for right clickers thanks to its attack speed bonus. If you are about to buy this boots, throw off one Ring of Protection from starting items and buy more Iron Branches.
Best choice I think. Movement speed is great for ganging in which you are good thanks to Reaper's Scythe. The armour gives you a lot effective HP, much more than Power Treads and HP regeneration comes handy at early game when you don't have high-level Sadist yet. Also I like that green colour. :)
Early Game Items
Ring of Basilius is often seen on Necrophos in competitive matches. It boosts your armour, helps you with mana regeneration (altough alone Sage's Mask gives you more mana regen then the whole ring) and increases your damage to help you with last hits. All of this comes in very cost-effective package. You can of course skip Ring of Basilius if someone of your team mates is about to build his or her own ring, e.g. Lycan, Broodmother or an agility carry. In that case skip Ring of Protection from starting items and buy more Iron Branches. It is also advisable turn off the ring when you are trying to keep creep equilibrium and turn it on when you ready to push.
Magic Wand is another classic early game item. You can skip it if you are doing well and want to buy early core item. But overall it is very good. Last patch 6.83 even increases its max charges to 17!
You are not obliged to buy both Point Booster and Vitality Booster at early game. You choose one or another depending on what you are going to buy later. If you are gonna buy Bloodstone then both Boosters make sense of course.
Middle Game Items
Bloodstone has very good synergies with your skill. It helps you spam Death Pulse when you do not have Sadist online and active yet. It is charged up when you get a kill and you are about to get a lot of kill thanks to Reaper's Scythe. And also It reduces your respawn time which is very handy. Also I like the fact that my opponents respawn 30 seconds later and I respawn 30 second earlier, which is the double time difference I can use to farming.
Bloodstone is good to get early in game but there are situations when you want to skip it. I used to think that only the closest hero with Bloodstone gets charges for a kill but it is not true. All heros in range get charge so it is possible to have more Bloodstones in a team. Main situation for skip Bloodstone is when you are in team with lots of healers and mana recharges like Io or Keeper of the Light. Second reason is if you're getting it slowly due to being behind. Third reason is when you want buy Rod of Atos. Stacking these items together is often unnecessary.
Blink Dagger is pretty common in competitive scene but not in a pub. But when I tried it in my pub games I cannot believe I played this hero without the Dagger. Firstly it helps you to farm faster. You can blink between camps in jungle or ahead on lane to spam Death Pulse on creeps. Secondly you can blink into team fight and spam Death Pulse which makes a huge impact. Also range of Reaper's Scythe is not the best one so with blink you get kills you normally wouldn't get. You can use Blink Dagger to escape when you feel you are in danger. The number of possibilities are endless. Competitive players buy Blink Dagger somewhere between 15 and 25 minutes.
Rod of Atos is the most underrated/overrated item in whole game I guess. In my opinion it can be pretty solid, particularly on Necrophos. For a mild cost you get an item with big boost to health and mana - i.e. resources you want the most. Its active ability has two purposes. Chasing enemy heroes and breaking theirs Linken's Sphere. You don't want to have your ultimate get countered by Linkens, do you? And with combination with Blink Dagger you can chase heroes on pretty long range. If you don't believe me go now and try it! As I said I usually don't buy both Bloodstone and Rod of Atos together. I just choose the one or another.
Aghanim's Scepter, the well-known item. You want to buy it around 30 minutes of game usually. It does not make much sense to buy it earlier but it make sense to buy it later. The power of Scepter lies in late game but you will benefit from extra health and mana as well as in mid game.
Late Game Items
Here comes wide possibilities of luxuries items. You already sold your Ring of Basilius and Magic Wand and looking for some more interesting options? The most common and most safe choice for late game is probably Shiva's Guard. You will become super-tanky and super-threatening in team fights. Very good item overall.
Another option is to upgrade your boots to Boots of Travel which frees you one slot for another item and makes you more present on the map, especially when your towers are gone already.
My favourite item when I am snowballing is Dagon. Blink into Scythe, then Pulse and Dagon's Energy Burst combo obliterate nearly anyone. With this you are able to perform some shiny rampage. But I advice to not buy Dagon if you are not snowballing.
Eye of Skadi and Heart of Tarrasque are solid tanky options if someone else already have Shiva's Guard. Beside not being right-clicker I tend more to Eye of Skadi. From my calculations I have theory that Eye gives you more effective HP then Heart (thanks to Agility-Armour boost). I also like the slow.
Refresher Orb is obscure item but occasionally it can be useful in very very late stages of the game when you need to remove two targets in one team fight. Ethereal Blade is theoretically icing after or instead of Dagon. You can also buy it when you purchase Ghost Scepter earlier to defend yourself against hard physical damage dealers like Phantom Assassin.
Items I don't mention
Mekansm used to be early core item on Necrophos for a very long time. Well it still is on general pub games. But with increased mana cost to 225 for activation it lost some of its appeal, at least for me and for competitive players as well (but their reasons may be different).
Ok. Now you are familiar with skills and items. What's next? The gameplay!
When game is starting you must pick a lane. With Necrophos you are quite flexible. You can go solo, midlane or just normal dual lane with some support. The last option is probably the most common. With proper support you have very high chance to score first blood and some early kills thanks to one point in Heartstopper Aura and few more points in Death Pulse.
Necrophos is hero optimized for position two in farming priority. If game goes well he often ended at position one and if game goes not-so-well he may decline to position three. But if you try play him at four or five position you simply waste his potential. So when I said at introduction he is a carry it does not mean he is the hard carry. E.g. if happens for you to be on lane with Medusa or Spectre, let her farm. You must be conscious. You role is to be sort of backup carry, utility and durable hero. You can help with gang, you can nuke, you can disable through spell immunity (so in emergency to stop some nasty channelling from enemies), you can push, you are good in team fights. Accommodate your items to the role which your team needs.
You goal is to farm one or two core items as soon as possible. For me it is the Rod of Atos or Bloodstone and Blink Dagger. Then the real team-fights and chasing/ganging may start.
In this section I choose some professional matches to illustrate how to play Necrophos according to this guide.
NaVi vs mYi: Funn1k Radiant Top Lane
I choose this match because commentators explain some things about starting items and two Ring of Protections. Funn1k maintain his second farm priority through whole match and he plays exactly like I suggest in this guide :)
Example how to recover from bad start due to bad item choices probably (he has no durability items at early stage). XBOCT starts with Black King Bar to protect himself from Skywrath Mage and lots of stuns. He ends game with Dagon when fountain diving.
VP.Polar vs Team Empire: Silent Dire Bottom
Nice game when we see an early Rod of Atos but sadly this game wasn't suit for it very well with Puck and Anti-Mage at opposing team (both have blink-ability to ignore the Cripple slow).
NiP vs Power Rangers @ SL-IX LAN Finals: Shachlo Radiant Bottom Lane