In the deepest, blackest pits of the ocean a fearsome and terrifying creature lurks. Bending the shadows to his will, few have seen his true form and none have lived to speak of it. For the shadows are his sanctuary, a place of empowerment and revitalisation. Bathed in this inpenetrable darkness he strikes unseen, every attack making him swifter and more powerful whilst weakening his enemy. An unbreakable leash awaits all who try to escape, leaving them at mercy of this horrifying phantom. By sacrificing a fraction of his soul, his pact with darkness allows him break even the most powerful bonds and curses. With echoes of the Great War reaching every corner of the earth, this creature, known only as Slark, has emerged from the depths to search for an ancient relic...
Slark is the ultimate hit and run hero. He has 2 forms of initiate and escape, he can outrun almost any hero, and he never has to go back to the fountain for health.
This guide focuses on the first build. The other builds are more flexible and mostly self-explanatory.
It seems a lot of people like to max Dark Pact (DP) first, but I find that Pounce is more effective (fully maxed) for level 6-12 ganking. Certainly get one level of DP early for getting rid of dust or Track, otherwise the aoe damage isn't fully utilized until you are strong enough to gank more than one player at a time.
Ring of Aquila packs a lot into a single slot, for an early game item. Stats are always good, and the mana regen boost allows you to use your skills more often w/o relying on fountain or bottle/runes (especially if someone else is rune whoring).
Power Treads (Strength) - Hands down the best boots for Slark. Health and attack speed are two of the most important things to make Slark viable.
The additional Ogre Club and Gloves of Haste are both cheap, and both build into core items. And again, add very important stats for Slark.
I moved Orb of Venom from situational to early core because I have been using it more often lately and find it to be very effective (for the price). Other than the obvious, it is very useful for when you cast Shadow Dance, as most heroes tend to start running. It also makes it easier to land your Pounce when an enemy hero is trying to juke.
You could also decide to get Mithril Hammer instead, if you are doing well. The neat thing about this is that you have the flexibility to build it into Maelstrom or BKB.
Also, if you are having trouble taking too much physical damage while trying to last hit, consider getting Stout Shield or Chainmail, which can be built into situational items. Grab Cloak if you are struggling with nukes.
An important thing to note is that all of these items are in the side shop (except Mithril Hammer).
Black King Bar - I strongly recommend getting it in every game except those that you are easily winning, or if the other team has very few disables. In this case, I would recommend getting some cheap hp instead, like a Bracer. In addition to the obvious benefits, BKB can allow you to win 1v2+ fights, whereas w/o it killing 2 players alone might be a stretch (assuming an even game). It also further increases escape ability.
Maelstrom/ Mjollnir (M/M) - This is actually the only reason I'm making this guide. None of the other guides headline M/M as a core item for Slark, but I believe it gives him more damage output than any other item. Looking at only damage per cost, M/M are both right up there with Daedalus, MKB, and Desolator. Add in the fact that Mjollnir has the most sustained attack speed for a single item, which greatly enhances your ES, and M/M seems a no brainer to me. Another benefit is the added aoe. Add lightening and static charge to DP, and you are putting the hurtin on everyone around you.
Hyperstone - Honestly, I often get this item before even finishing Maelstrom or BKB. Attack speed is just that important for Slark.
These are obvious, stuns and damage are always good.
Helm of the Dominator or Vladmir's Offering - I generally don't get lifesteal with Slark, as he is not intended to stand toe to toe against a typical lifestealling hero. However, if there is no better carry on your team, and you are struggling against an enemy carry, it might be necessary.
Diffusal Blade - I only get this against certain heroes: Omniknight and any hero with very strong dispellable bufs (ie Ursa, Troll Warlord, etc).
Gem of True Sight - The best counter against an invis hero, is an invis hero with a gem. Also good for finding wards and mines.
Heaven's Halberd - Very useful against other glass cannon heroes, hp heavy carries, and passive stunners. More health, evasion, and four free seconds of damage and stat stealing and you should no longer have a problem.
The rest are for utility and/or survivability.
Heart of Tarrasque - Your ultimate gives better regen than heart, don't get it.
Hand of Midas - Slarks natural role is a ganker, so Hand would largely go wasted. You will get more gold and xp from proper ganking than farming.
Mask of Madness (MoM) - I can't tell you how many times Ive seen a Slark with MoM get ravaged by a nuker. Sure it works if you have the stronger team, but in an even game you will find yourself dying more than you should.
Desolator - While I am a big fan of Deso, it doesn't synergize with ES nearly as well as the other dps/luxury items I've listed.
I'm sure there are people that can solo a lane with Slark, but I wouldn't suggest it. With a lane partner you are more likely to get more creep kills, less likely to die, and more likely to get hero kills. Slark is honestly pretty weak for the first 5 levels. I generally prefer the jungle lane, so you can jump on a neut node (after you are level 6) if you expect some down time.
As for good partners, any hero with disables or an attack speed buf can greatly improve Slarks early game. The more ES that Slark can get in, the more likely you will get a kill. In the early game, 10-20 damage (3-6 hits) can make a very big difference in a lane fight.
Another good partner are typical Vlads users. Though I don't think its worth my gold to buy lifesteal, it can certainly be useful in team fights.
A couple of heros you don't work well with are Anti-Mage and Necrolyte, because your ES lessens the damage of their ults.
Heroes that that you should be afraid of are those with heavy nukes ( Lion, Lina, Lich, etc), or stun/holding spells ( Earthshaker, Shadow Shaman, etc). You should also be wary of heroes that can keep up with you ( Bloodseeker, Spirit Breaker, blink heros, etc) and keep you from getting your ult passive. Beastmaster's hawk is also very annoying and even debilitating if used correctly.
One hero I always have trouble with is a good Tiny. If he has the mana, he can one hit you (even when you are invisible) unless you really build up your hp or get spell resist. Also, being stunned just for hitting him is always annoying. Honestly its best just to avoid him unless its a team gank and you don't initiate.
One game I had against a Tiny, he and another hero came to gank me. I immediately Pounced and casted DP(to cancel tinys dust) and SD and ran. My DP canceled the dust, so I got a pretty good distance away, but Tiny had tossed the other hero (an insane distance), who then immediately dusted and stunned me. Tiny blinked in and I was soon dead.
Timbersaw can also be a pain. He has the mobility to keep up with you, and all of his skills can pretty easily hit you even while invis. A good Timbersaw will have the mana to spam his skills, and you won't stand up to them for very long. He can also be pretty tanky, so you usually can't take him down as quickly as other heros.
I'm usually very wary of team fights because by that time I often have many kills and the other team will focus me. If this is the case, I will move parallel with my team, but out of sight (like in the jungle or on the other side of a cliff). Once the fighting has started (all their main skills are on cd), I try to go in behind the enemy and take out the supports first. If I draw too much attention I either cast SD, cast BKB, or get out of the fight and come back in once they start focusing on someone else again. Also, make sure to cast Static Charge on an ally that is taking some heat.
I know I didn't cover everything, but I feel that the rest of the guides do a sufficient job covering everything else.
Slark shines at getting in fast, killing fast, and getting out fast. He does best when the enemy doesn't know where he is, and don't see him coming. Later in the game the ideal strategy is to catch one or two enemies alone, build up ES, take several seconds to regenerate, then hurry to the next fight before a minute is up. In this manner Slark can sustain an extremely high amount of damage without a reliance on mana or strong items.
Let me emphasize that the main thing Slark needs are health and attack speed. Any items with these stats will work, but I am proposing that this build maximizes his potential. The main idea is to get the most out of your gold and item slots. With this build (+/- some situational items) I have averaged 18-5-10 in 10 games.