Slark

Slark the Nightcrawler is a melee agility hero with extremely high mobility and damage potential, capable of stealing stats on auto-attacks to boost his own damage while remaining unseen even while attacking. His Dark Pact is an area-of-effect damage spell that releases dark pulses at the cost of his own HP. This spell also doubles as a self Purge, removing negative debuffs from Slark such as slows, stun, etc. Pounce, his second ability, causes Slark to leap forward, binding the first hero he hits to the ground. His passive skill Essence Shift is what makes Slark fearsome when unattended - each of his attacks steals one point of Agility, Strength and Intelligence from his target and converts it into bonus Agility for himself, thus increasing his damage, attack speed, and armor. Slark's signature ability, Shadow Dance, gives him invisibility, although unlike most Invisibility spells, Shadow Dance doesn't reveal Slark even when attacking or casting spells; it also passively gives him bonus movespeed and health regeneration as long as he is not seen by enemies.

Tips

 * If you are low on HP after level 6, 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. The increased move speed from Slark's ultimate will allow for easy escaping.
 * 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.
 * Dark Pact can purge debuffs such as Slardar's Amplify Damage, Dust of Appearance and Bounty Hunter's Track.
 * Gem of True Sight, Lvl 3 Necronomicon and Sentry wards are arguably the best counters to an invisible Slark.
 * 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.
 * Your invisibility gives off a smoke visual that will alert enemies that you are present, but they can't see you. This could be a big drawback to Slark's potential, as there's a chance that he will be revealed. Be wary and cautious when facing heroes, especially those with Dust (although it is purgable by Dark Pact), Gem of True Sight and a Lvl 3 Necronomicon.
 * In teamfights, it's ideal to kill the tanks first since they have more HP - you will have more time to steal stats with Essence Shift, giving you an advantage against rival carries easily if you can't beat them outright. Slark is a powerful counter against tanks like Axe, Tidehunter, and Pudge.
 * However, the aforementioned heroes also have powerful spells that target an area around them, Counter Helix, Anchor Smash, and Rot respectively. A Slark should take care when attacking one of these heroes, lest they turn the tides on the Nightcrawler and counter gank him.
 * Slark functions a bit differently from other carries - while other carries capitalize on sustained damage, Slark takes on a more hit-and-run approach. Abuse 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 his Essence Shift by taking objectives using his stolen stats.
 * There are two main skill builds for Slark: one of them is based on maxing Dark Pact before the other skills; 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 great addition to his skill set for both set-up and escape from ganks. The twelve second invisibility also provides a means of hp regen after taking damage from team fights without being forced to leave the area. Allowing you to mop up any retreating heroes for some extra gold and exp.
 * If timed right Pounce can be used to dodge some projectile skills such as Sniper's ult.

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,  Drow Ranger, and  Luna (with Lunar Blessing), is one of the few heroes to have 1800 sight range at both day and night.