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 stats 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 stats 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.
 * It is important to remember that Dark Pact can purge True Sight debuffs such as Dust of Appearance, Slardar's Amplify Damage, and Bounty Hunter's Track, which would otherwise disable the active bonuses of Shadow Dance and cripple you in the event of an escape.
 * 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 is 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 couple of attacks 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 the 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.
 * Try to 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 your team has no natural counter for an enemy Slark's invisibility, consider Sentry Wards, Gem of True Sight, or a rank 3 Necronomicon. Unlike Dust or some hero spells, these cannot be purged by Slark's Dark Pact.
 * 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 two 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.
 * Slark functions a bit differently from most carries - while a standard carry capitalize on sustained damage, Slark takes on a more hit-and-run approach. Play 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 stats 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 stats. 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.
 * While not mentioned in the default build, Shadow Blade makes a somewhat decent addition to his skill set for both set-up and escape from ganks - the twelve second invisibility provides a means of health regeneration after taking damage from team fights without being forced to leave the area (sans team fights in sentry-warded areas, of course). It is rarely a viable choice, however, since it has more drawbacks than benefits on Slark - his ultimate's active renders the item's active redundant, it does absolutely nothing to improve his defenses, and weighs him down with an item slot that further highlights his weakness to True Sight.

Trivia

 * 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, along with Enchantress,  Meepo and  Luna (with Lunar Blessing), is one of the few heroes to have 1800 sight range at both day and night.
 * Slark's response upon activating a Double Damage rune, "Met you once, cut you twice," may be a reference to the popular Design Technology phrase "Measure twice, cut once"