From a counter-craft standpoint, N'aix is the quintessential anti-tank. He has built in sustain and deals damage based on the HP pool of the target. From a balance standpoint he is very strong in the lane and in the early game, making him a beloved pick of many top teams in the current version.
Strong against: Lone Druid, Spectre, Alchemist, Axe, Keeper of the Light
Weak against: Faceless Void, Phantom Assassin, Templar Assassin, Antimage, Puck, Beastmaster, Shadow Demon, Weaver
Notably N'aix is not strong against heroes such as Slardar and Spirit Breaker. Bash and armor reduction make minced meat out of the Lifestealer.
Now because N'aix is single target and has gap-closing issues, does not naturally have a lot of output, does not farm quickly, and has a limited window during fights in which to deal his output, this hero is not very suitable as a single-core or often even as the main carry in organized play. His greatest strength is in his early magic immunity and the pressure he can provide on the lane and during the early game. Because he excels in the early game small skirmishes, he is especially suitable for AP games.
Skill Build: EQW, then max Q followed by E. Rage (Q) is your main fighting ability and maxing Open wounds afterward reduces the cooldown so you can engage in many small skirmishes back to back. One point in Feast provides the bulk of the hp% bonus and additionally in the early game heroes are not that tanky yet.
Item Build: Phase boots, Wand, Drums, Armlet, -damage item-, -sustain item-, Abyssal Blade
Phase boots are better than treads for several reasons. The chief reason is it aids in gap-closing so we can get to our target. A second very important reason is that the benefit of treads over phase boots from a DPS perspective does not kick in on most heroes until level 10+. N'aix is an early game hero with relatively low natural output and phase boots provides much more damage. A third good reason is that with Rage we already have a large attack speed increase but we are missing raw damage. (and increased attack speed (IAS) has diminishing returns).
Wand and Drums for stability and base stats, not much needs to be said.
Armlet, along with Maelstrom are the two main medium-cost items that are arguably among the most cost-effective items in the game. The benefits of armlet for strength heroes are immense, but the chief problem is how you will sustain yourself after the health loss. For N'aix this is no issue.
After Armlet, it is advisable to build some type of fighting/damage item and then sustain. For all heroes, the build order principle of output then sustain remains. It is unforgivably useless to tank up if you have no output, especially if you are a single core. The current main line after Armlet is Sange and Yasha solely due to the influence of YYF. It provides good stats, can be bought in pieces, and with it you approach max movespeed. This suits N'aix's playstyle of getting in and getting out quickly and often.
My item of choice is Desolator. Its cost is about the same as SnY but provides much more damage and complements feast which is physical. It aids in tower sieging and damage from all allies will be amplified. The way I view Lifestealer, you have a limited time to do as much damage as possible. You already have movement speed and attack speed but what you are lacking is damage.
Afterwards your options are Heart and Assault Cuirass. Heart allows you to keep Armlet on (which really changes almost nothing but it makes your life easier), and fits very well with N'aix's fight style of go in, do damage, go out and heal. For taking high ground by attrition, this is the best item. Every time rage is off cooldown you can go in and get a few whacks.
For strength heroes, from a 1v1 standpoint, Heart also does not lose to Assault Cuirass, especially when we already have attack speed.
However in situations when I feel I must go into a fight and come out on top and when the other side has armor reduction, such as against TA or Nevermore, I choose Assault Cuirass over heart. The armor reduction will also stack with your Desolator, giving you more bang for your buck. (Armor provides diminishing returns and inversely, armor reduction compounds).
So much for build discussion. As far as fighting goes this hero is a little bit involved. Phase boots' active is canceled if you use any spell or item, so you have to think about what order to use everything in. Most typically it's open wounds, phase, then turn everything else on. By the time open wounds runs out, phase cooldown will be off again, but so will Rage. At this point you can either continue fighting or back out until you have Rage again.
As for which target to choose in a fight (and we hope you are not a single core), this also requires some thought. You can either chop at the front tanks and carries or use your magic immunity and go for the back line. Conventionally, you are supposed to go for the back line and take out the supports, even though you are a so-called anti-tank. I would however advise just going for whoever is closest and whom the rest of your team is focusing.
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