This is a very quick guide to "N'aix, The Lifestealer". This is not very detailed but just gives you an idea of how to play this hero. Some would suggest otherwise but this build is tested by me personally on different situations and i would just like to share it with you.

Team Role:
Early Game Role: Carry Farmer
Mid Game Role : Carry Farmer
End Game Role : Pure Carry

Carry Farmer:
Though N'aix has a slow, and a good one at that, he wouldn't be able to contribute much of his feared dps early game. Ganking early with this hero could also lead to him having some unneeded deaths. He needs his items to be able to quickly kill an opposing hero within the duration of his slow. Farming well is important to this hero to be able to be effective in teamfights.

Carry Farmer:
I still cannot say that he can be a solid ganker at this point in time. You should have AoM by this time so your dps would be good enough. Having dagger by this time would give you the positioning advantage to maximize the duration of your skills. Still, you cannot afford to die from ganking as you still have a lot more items to purchase to go "Rampage!" during clashes. And he is very capable of doing so. Focus more on farming neutrals and defending towers to gain more gold safely.

Pure-carry
After obtaining a damage item like Desolator and/or Cranium Basher (he is one of only few that can really make good use of this item) you can proceed to raping your opponents. Use dagger to position yourself near your target, cast open wounds on him, click rage and armlet, get the kill, choose next target and get another kill. This guy can really be scary with all of the said items. After spending the early stages hiding and farming, it's your turn now. Make them fear you.

He is also an anti-tank but i didn't care to put it in his role 'coz even if you can easily bring down tanks you shouldn't really focus on killing them first.

Item build:

1

Justifications:
You need to attack fast and the +8 str from treads adds to your damage and survivability. The attack-speed bonus allow you to gain back life from neutralling thru lifesteal. This is a no-brainer.

Urn of Shadows is a good upgrade for the 2 gauntlet of strengths you purchased as a starting item. The mana regen is needed to spam rage and the healing effect it gives you after killing a hero is very valuable. If a support teammate gets this item you may choose not to get this anymore. It helps especially since your hp would most probably drained after using armlet.

Naix is the best Armlet of Mordiggan carrier. It's like, this item was created with Lifestealer in mind. This item gives everything he lacks and his skills make up for the degeneration that comes from activating it. Definitely a must on this hero.

Kelen's Dagger of Escape is an important positioning item for heroes that need to maximize the duration of their dps increasing skills. "Decent players can do without this, Decent players become Great players with this".

Stygian Desolator gives you the damage you really need to be able to make short work of your target. It is very efficient for its price in terms of increasing damage.

Extension builds are up to you. That's where your creativity and good assessment of the situation should come into place.

Honorable Mention:
1.

There are times when i would suggest you also take Cranium Basher especially against blinking or leaping heroes and heroes that can go toe-to-toe against you.

Skill build:

Lvl 1 Rage
Lvl 2 Open Wounds
Lvl 3 Feast
Lvl 4 Rage
Lvl 5 Feast
Lvl 6 Infest
Lvl 7 Rage
Lvl 8 Rage
Lvl 9 Feast
Lvl 10 Feast
Lvl 11 Infest
Lvl 12 Open Wounds
Lvl 13 Open Wounds
Lvl 14 Open Wounds
Lvl 15 Stats
Lvl 16 Infest
Lvl 17 - 25 Stats

Justifications:

Rage and Feast are the most important to max early. You need this to quickly devour neutral camps. One level of Open Wounds is enough early game. Putting levels on it only decreases the cd time and increases lifestealed. The slow duration is consistently 8 secs so one level is good enough. Besides, you're supposed to be farming during early stages, right? Infest is not the best skill around but there are a lot of situations where you can use it to its potential. This skill makes N'aix a fun and not so boring hero to use. Be creative with that skill.

Good Allies

Anybody with a decent disable is a good ally to every hero so I would mention only a few with great synergies with him.

This hero more than just serves the plate for N'aix. He basically served it with bottomless iced tea. Casting stun and slow on the target is enough. Giving N'aix an attackboost is simply too much.

With the ability to slow, heal and increase survivability makes this hero N'aix's bestfriend. You'd be thanking Dazzle from start to end.

Weaknesses

Nothing f***s you up more than an item that wastes your rage (avatar) duration. If you can't kill it quickly, you might not be able to kill it at all.

N'aix at the Competitive Scene
Rarely picked 'coz of too much farming needed. Also needing a great support player so that his target won't escape. Used as a pure carry, this hero is a beast during rage duration. Can take down any hero without a "blink kind of escape mechanism" and tanks in just a couple of seconds. Once the rage buff duration is done, though, he reverts back to a simple str-based hero. The short cooldown on rage makes-up for it though. When you see him get picked it means that his team is ready to go 4 vs 5 during the early and mid stages of the game. But late-game is where he really shines. Well, like what they say in life, "You have to sacrifice one thing to get another."

Where can I find replays?

I suggest you go to Dota Commentaries.com as they don't just give you replays, they also give some analysis of the item builds, warding and team play. Not to mention, they are commentating competitive games and not your ordinary pub games where any build would work.

If this guide has helped you in some way, please feel free to comment and/or rate my mini-guides. Thanks a lot!

Good luck! And remember, reading guides gives you ideas, but, only thru constant practice does one achieve greatness.