Slark is a mid- to lategame semicarry who can deal much damage in a blink of an eye and regen up even faster.
He lives from ganking and can roam very early on, even in the enemy half of the map.

Slark falls of a bit in the lategame and will be easily countered through sentry wards and a gem, most other things he can just dispell.
This build is a decent, yet not perfect itembuild and annotations containing my own opinion. This my not be the common Slark, but this build helped me often to win the game.

I recommend to play him in the midlane, because you will get fast levels and the basic gold you need to gank.
Other lanes work aswell, but it is hard to adapt his playstyle as a carry, because Slark is a heavy ganker and lives from kills and fighting.
The Offlane is pretty scary, you are very squishy and your jump isn't an escape when you hit your enemy, it is possible if you are matching versus few stuns and sqhishy/weak enemies.
I will go more into the midlane in this guide, because in my opinion it is the best lane you can pick up.

After blocking the creeps, try to get two or three hits on your enemy, the Essence Shift allows you to weaken him aswell as giving you some bonus damage, what comes in handy when you try to lasthit and deny creeps.
When you hit level three to four you can easily kill most of your midlane enemies, get some hits on him, pounce him and finish him off with some last autohits. That works on most of your matchups, don't forget your q to dispell CCs.
Get your bottle as fast as you can, next items should be boots of speed, an orb of venom and a tp scroll.

As soon as you hit level 6, don't waste time in your lane. Search for someone to gank, preferrably the carry, but Heroes like Anti Mage or other ones with a blink ability are hard to gank without a proper lockdown, since they can just get out of your Pounce binding.
Continue to harass and kill the enemies so your carry can get freefarm, if you need help try to get the supporters involved.
Even when your laning phase got ♥♥♥♥♥♥ up and you lack of levels, Slark can catch up because of his strong ganking abilities, so don't give up too early!

Even when Slark is an AGI Hero and can carry, he falls off in lategame.
To be Hardcarrylike, you need a ♥♥♥♥ ton of items, otherwise you wont have enough damage and susatin in teamfights, furthermore you don't have a huge AoE or lockdown to control targets, you are really dependent on a basher, orchid or similar to comepensate that.
Try to get Objectives with your team, splitpush and don't be the first to get in the teamfight, also be careful of truesight, which will pretty much disable your ultimate.

Good Luck!