Slark, the Slippery Devil

Introduction

Hello, I am E4rl, an average DOTA 2 and Slark player, and I have a different playstyle from most Slarks. Please note that this is my first guide, English is not my primary language and I'm not a pro player.
I know that everyone has different opinions about builds/playstyles, and I'm not implying that this is the best way to play as Slark.

Pros and Cons

Pros

-A killing machine if used properly.
-Has 2 escapes, a nuke/purge, an awesome stat-stealing passive and a huge HP regen.
-Highly mobile ganker
-Great Solo-Killing potential
-Can carry relatively well
-Not really gold dependent
-Can detect wards/invisible enemies with Shadow Dance's passive

Cons

-Level dependent
-Squishy before level 6
-Can be outcarried Late Game by hardest carries
-Mana hungry early-mid game

Skills

Pounce is Slark's most important skill, and should be maxed first after taking an early point in Dark Pact and Essence Shift. It can be used as a nuke/snare or as an escape. You should always max this first because the decreased cooldown can save you in most situations, and it hurts a lot when at max level. Use it to initiate the gank or chase a fleeing enemy. If you are facing the right direction and know how the skill works, you'll always hit it. Also, if timed right, Pounce can be used to dodge projectiles such as Sniper's ultimate.
Dark Pact is also really useful for removing debuffs/stuns(When timed properly), and should not be used as a nuke before maxing it. I personally max it last because I don't really need the extra damage in the early-mid game (And it drains the precious mana that should be saved for pounce), and only use it to purge debuffs/stuns before late game, when I have enough mana to use it to nuke AND purge.
Note: The 50% damage you take from a maxed Dark Pact can make a huge different in the early game.
Essence Shift is an awesome passive that should be maxed second (So you can get mass agility after some ganks). When maxed, after a large teamfight, it is not uncommon to steal 50 or more agility, and that allows you to make quick work of untended towers or Roshan.
Shadow Dance, an awesome ultimate that can be used offensively and defensively. Do you need some extra movespeed to chase that low HP carry that is running away with Phase Boots? Shadow Dance then Pounce and he's dead. Do you want to tower dive without taking ANY damage? Shadow Dance. Do you need to tower dive to get that kill? Shadow Dance. Did you accidentally Pounce in the middle of 5 heroes? Shadow Dance... There are lots of situations where this skill is useful. Also, it has an awesome passive, that gives huge HP regen and movement speed when the enemy can't see you. A helpful effect of your ultimate's passive is that you gain a weak form of detection. If you find that you've suddenly lost your passive effect while seemingly alone, it can mean an enemy is lurking around in the area and you can't see him or there are wards nearby.

Items

Starting Items

Stout Shield: This helps a lot in lane, specially if your enemy is harassing a lot.
Tangos and Iron Branches don't really need explanation: Regen and GG branches are always good.
Also, if you're feeling agressive, you can buy only the set of Tangos and the Stout Shield, then immediately send the courier to the Secret Shop and then buy an Orb of Venom after farming that 25 gold.

Early Game

Bottle: Very important for solving your mana problems from spamming Pounce, and when you're mid laning, it's a must: Rune control and gank!
Poor Man's Shield: Really useful if you are taking a lot of harass in lane, and gives you extra agility!
Magic Stick: That burst heal might just be what you need to get enough mana to use that decisive pounce. And it will save you lots of times, specially when against spell spammers like Bristleback. (Only upgrade it to Magic Wand against spammers like him)
Boots of Speed: Do I really need to explain?
Orb of Venom: The slow is minor, but in early game, it matters. Also, it builds into Eye of Skadi later!

Mid-Late Game

Power Treads: In my opinion, the best boots on Slark. It gives bonus attack speed, and that means you'll steal more agility. Also, there are lots of benefits from tread switching. Only get this before Yasha when you've got 1-2 kills in the early game.
Sange and Yasha: Gives more agility and strength, good bonuses for Slark, as you will hit harder and get tankier. Also, it has an unreliable slow and movement speed bonus. That helps A LOT with chasing, as Slark is kind of slow when enemies can see him.
Skull Basher: In teamfights, when you have stolen enough agility, this can be used to stun lock your enemies. Just Pounce in their faces and start hitting them like a madman: They won't even be able to react.
Eye of Skadi: Gives you some bonus health and mana, and a really nice slow that helps even more with chasing. When you finish farming for this, you'll be kicking ***. Do not get it before Skull Basher! Skull Basher is cheaper and a lot more useful in the mid game.

Super Late Game (Luxuries)

Butterfly: Nice evasion and agility bonuses: A really good item on Slark, but should only be gotten after his core items.
Abyssal Blade: Nice strength/damage bonus, but the passive/active are the best parts. STUN LOCK THEM FOREVER!

Situational

Black King Bar: If have lots of stuns and your skills aren't enough to keep you alive, get this.
Orchid Malevolence: Tired of that Anti-Mage who just Blinks away when you gank him? Just use this after a sucessfull Pounce and he'll probably die. Also, it helps a lot if you're facing someone who just stuns/slows/silences you and runs away when you Pounce on them.
Important Note: DO NOT get this to counter Nature's Prophet's Teleportation. Get a Skull Basher, it's less expensive and more effective.
Heaven's Halberd: Useful when you're against that Ursa who always turns around and kills you when you Pounce on him.
Medallion of Courage: Do you enemies try to run when you Pounce? Get this. (Only on the early game)

Rejected

Mask of Madness: Any kind of Lifesteal is horrible on Slark, as you can just run away from the fight for some seconds then come back with full HP. Also, the 30% extra damage that you take really screws up with you, as you're squishy. If your enemy has brains, they will nuke you to death.
Vanguard: An horrible item for all heroes. Never get it. Also, you don't need to be tanky, as you can escape easily from most situations.
Blade Mail: You should be avoiding damage, not reflecting it...
Radiance: It is too expensive and you just don't need that AoE damage.
Heart of Tarrasque: YOU DON'T NEED TO BE TANKY! And you already have huge regen.
Phase Boots: You don't need the extra damage + movement speed on early game. And by mid game, you should have at least a Sange and Yasha.
Mjollnir: This is an horrible item. You don't need the attack speed, you have Essence Shift. Also, this is better on farming carries. And answer me: ARE YOU A FARMING CARRY? If you said yes, stop reading this guide and go play as Alchemist. You need no items to farm creeps, you farm heroes.
Armlet of Mordiggian: Seriously, Armlet on an agility carry? SERIOUSLY?
Shadow Blade: You already have lots of escapes, and you don't really benefit from the extra damage, because you should be pouncing before attacking. And about the "Permanent Haste": It is temporary, and you get the same thing and more benefits from a Sange and Yasha.
Drum of Endurance: Let your supports buy it, and get your Sange and Yasha. It is a lot more useful and just a little more expensive.

Gameplay

Laning Phase

Try to solo a lane if possible (preferably the middle lane), so you can get gold/level advantage. Always gank when you have a chance, but don't get overly agressive before level 6.
Slark is better soloing but works on a dual/trilane, specially with heroes who have stuns/slows.

Mid Game

When you reach level 6-8, you should start ganking like a madman. Leave your lane for someone else, and go farm on heroes. Do not avoid teamfights, you're specially powerful in them if you manage to kill weak supports and steal enough agility to cause some serious damage (If your team lacks an initiator or the initiator lacks a Blink Dagger and can't position himself properly, you can initiate by Pouncing and Shadow Dancing. This might only work on this stage of the game). ALWAYS push after sucessfull ganks and teamfights, start farming for your core, and KEEP GANKING. Also, you can help with dewarding if you explore your ultimate's passive.

Late Game

If your early-mid game was really good and you ganked madly, you should be winning and your Hard Carry (or maybe you) probably has farmed a lot and finished the core items. At this stage of the game, you just need to finish farming for your Luxuries, then go pushing/teamfighting all time.

Foes, Mates, and Free Kills

Foes

Bloodseeker: Thirst will reveal you when you're low on HP, so your ultimate's passive won't work. However, he isn't a real threat to you in ganks/teamfights. You can just Dark Pact his silence away (With proper timing) and use Shadow Dance to nullify Rupture. Just gank him a lot in the early-mid game: DO NOT LET HIM FARM. An averagely annoying hero, specially if he was fed or farmed well.
Ancient Apparition: He isn't a real threat, but his global ranged ultimate is really annoying, as it doesn't let you regenerate through Shadow Dance's passive. (I still don't know if you can purge the ult with Dark Pact, but I guess you can't.) Besides that, he's squishy and can be killed easily if you catch him with a nice Pounce. Averagely annoying too, but really easy to kill.
Shadow Shaman: A good Shadow Shaman player would force you to use Dark Pact, then use the other disable on you. Like Ancient Apparition, he is squishy, so you can kill him easily. Not so annoying and easy to kill.
Lion: Same as Shadow Shaman: A good Lion would force you to use Dark Pact, then use the other disable. He's squishy too, but Finger of Death can screw up with your little HP pool.
Doom: Well, his ult counters everything! It can even disable your ultimate's passive.

Mates

Kunkka: He can aim his Torrent and Ghost Ship easily after your Pounce.
Leshrac: Same as Kunkka. He can aim his Split Earth easily after your Pounce.

Special Mention: Io + Slark

Io Tethers Slark, who Pounces on the enemy. Io goes around the enemy and they get slowed by Tether. Also, Overcharge + Spirits can greatly increase the damage done. Do not forget to Relocate gank often.

Free Kills

Squishy supports or carries, like Crystal Maiden, Sniper and Drow Ranger.

Nice Tip: Always gank that Sniper and that Drow. They deserve a punishment for ruining everyone's fun with such easy heroes.