Slark

Slark the Nightcrawler is a melee agility hero that utilises his skills to spring onto enemy heroes and slip out unhindered. He is a very mobile ganker, but remains statwise below most other carries unless he is able to steal away attributes with his abilities. Once he does, though, only a few heroes can hope to be as fearsome as the Nightcrawler - extremely mobile to the point of ever-presence; his strikes only hitting harder and faster.

Slark can quickly leap forward onto enemies with Pounce to leash them and bind them from escaping. His Dark Pact releases a delayed purge that breaks even the most powerful disables. This gives him one of the most flexible abilities to start and end fights in the game. He is also capable of stealing attributes on auto-attacks to boost his own damage temporarily with Essence Shift which can rapidly shift engagements in his favor, even when merely performing hit-and-run maneuvers. Slark's ultimate, Shadow Dance, allows him to remain unseen even while attacking for a short duration and allows him to move quicker when not under the watch of enemy eyes to strike in and out of enemy ranks unhindered.

Tips

 * Pounce is good for both chasing and escaping enemies.
 * Dark Pact is a wonderful tool for purging the negative debuffs placed on him. When used at the right time, during the delay, the waves emitted from Slark will also remove the incoming debuffs about to be placed on him, allowing you to "dodge" or shorten their duration.
 * Unlike most purges, Dark Pact can purge stuns from Slark - bear this in mind if you have been targeted by a slow moving projectile stun (such as Sven's Storm Hammer or Alchemist's Unstable Concoction) and time your activation of Dark Pact to remove it as it strikes. With enough practice you can time it to almost completely nullify their usage.
 * Dark Pact also offers Slark a very interesting kind of advantage against certain defenses. The burst from Dark Pact is calculated as 10 attacks very quickly after another, meaning that Slark can use it against buffs like Living Armor, which offer protection but only defend against a set number of damage instances before disappearing.
 * If you are low on HP after learning your ultimate, Shadow Dance, try to avoid being attacked and then take advantage of your ultimate's passive healing ability.
 * When being chased, try to get out of sight from the enemy, be it around trees, up hills, et cetera. The passive speed boost from Slark's ultimate will activate when out of enemy sight and last for a short moment after sight is regained. If you are desperate, make use of Pounce to quickly clear treelines and cliffs.
 * You can take advantage of the bonus attack speed from both Mask of Madness and Essence Shift but be wary of the increased damage taken as Slark is very squishy.
 * If timed right, Pounce can be used to dodge some projectile skills such as Sniper's ultimate.
 * A helpful side effect of Shadow Dance's passive is that Slark gains a weak form of detection. If you find that you've suddenly lost your movement speed and health regeneration bonuses while seemingly alone, it means one of three things. It can mean an enemy is lurking around in the area that you can't see and you should be extra cautious. Otherwise, if your enemies are all accounted for, it means the enemy has wards in the area and you should let your team know or Bloodseeker has skilled his Thirst on you while you are below half health.
 * Slark functions a bit differently from most carries - while a standard carry capitalizes on sustained damage, Slark takes on a more hit-and-run approach. Maximize his Essence Shift's duration by continually harassing and retreating. Go for the kill once you think you have enough damage and attack speed to take down enemy heroes. Slark's base attributes are average at best, although with enough stolen Agility he can keep up with hard carries such as Phantom Assassin and Sniper.
 * Maximize Essence Shift's efficiency by taking objectives using his stolen attributes. After a large teamfight, it is not uncommon to find that Slark has stolen upwards of 50 agility, making quick work of untended towers or Roshan.
 * There are two main skill builds for Slark: one of them is based on maxing Dark Pact after maxing Pounce, with one point in Essence Shift; this way, you'll have a good nuke with a very low cooldown, which works perfectly with your Pounce. The second build is based on his passive ability, Essence Shift: the more you attack enemy Heroes, the stronger you'll get, since you'll gain more and more agility.
 * Soul Ring provides a cheap source of instant mana, and Shadow Dance can easily regenerate the health cost.
 * Orchid Malevolence offers all the mana regeneration you will ever need, bonus attack damage and speed, as well as the damage amplifying Silence ability. Together with Pounce, it will lock down any target almost completely. Note, however, that this item choice does not supplement your naturally low health at all.
 * Shadow Blade's stealth offers great survivability and escape, as well as allowing you to land surprise Pounces.
 * A good alternative for a Unique Attack Modifier (albeit a less defensive one) would be the Diffusal Blade. While it may not provide him the same survivability as the Eye of Skadi, it grants excellent Agility attribute boosts, 8/16 (Lv1/Lv2) total uses of the Purge active, and the signature manaburn attack modifier, which is a good countermeasure against spell-dependent foes.

Trivia
Slark's response upon respawning "Griefer madness" may be a reference to one of the worst films in cinema Reefer Madness
 * The DotA version of Slark bears many similarities with a rare World of Warcraft creature of the same name.
 * Slark's murloc heritage was completely removed in transition to Dota 2 to avoid copyright issues, since the murloc are an existing race in the Warcraft universe.
 * Slark's response upon activating a Double Damage rune, "Met you once, cut you twice," may be a reference to the phrase "Measure twice, cut once"