Spectre

Mercurial the Spectre is a melee Agility hero adept at dealing pure damage to lone targets who is powerful both offensively and defensively. Spectral Dagger provides a significant amount of mobility, allowing Spectre to go through objects, units and terrain while slowing foes that come in contact with the shadowy path left by her Spectral Dagger. Desolate allows her to inflict high pure damage to enemy heroes when she catches them alone. Dispersion reduces and reflects all damage, whether by attacks or spells, to an area around her as pure damage. Dispersion gives her great durability, especially when equipped with items that provide health, armor, and damage block. Finally, her Haunt creates a malevolent spectral illusion to all enemy heroes in the map. Haunt lets Spectre wreak havoc in clashes and ganks, while her allies take advantage of the confusion. Spectre can use Reality during the Haunt, instantly teleporting her to her illusion to take its form. This lets her hound her victim anywhere on the map. Mercurial is a dangerous supernatural being, relentless as she can chase and stalk down her victim to a global range, unhindered by boundaries. Her kit allows her to play the role of the team's hard carry by applying an immense amount of pressure in late game team fights. More specifically, the pure damage reflection from Dispersion makes it disadvantageous for teams to focus her. Her abilities have a complex nature, and newer players should generally not play Spectre as she is extremely farm and level dependent. It is best to face Spectre with allied help and strength, as encountering her alone isn't always the best option.

Tips

 * The shadow path left by Spectral Dagger lets her keep track on her prey. It also slows her target and enemies who came in to the path.
 * Spectre has a weak early- and mid-game, as well as a lack of lane presence. Picking Spectre requires a team that will largely be able to hold out for a large portion of the game until you have accumulated enough major items to start fighting, so keep this in mind when building your team.
 * It is generally a good idea to level up Spectral Dagger first to provide Spectre with extra mobility so that she is harder to pin down during ganks. However, be wary of its mana cost, as a single Spectral Dagger will typically consume half your mana pool per cast (in the early game).
 * It is sometimes beneficial to leave Haunt at level 1 in the early- and mid-game. Your illusions will not be strong enough to significantly benefit from an extra one or two seconds during skirmishes or team fights, and the extra points may be better spent in your other skills or stats.
 * In late game teamfights, Haunt and Desolate will allow your illusions to wither down support heroes at the back at significant speeds. In addition, you should always be in the middle of team fights to maximize the damage reflection of Dispersion.
 * Spectre can perform as a viable solo offlaner by using Spectral Dagger to hide in trees before creeps meet in lane, allowing you to freely leech EXP off the lane with no risk of being killed, and quickly reach level six to gank with Dagger and Haunt.
 * Mjollnir adds significant farming and push potential, while filling the AoE damage role of Radiance while also scaling better and having an easier buildup. However, illusions get no benefits from it.
 * Eye of Skadi provides great general stats, solves Spectre's mana issues, and gives her a slow that penetrates through magic immunity that allows her chase and forces attention on her. The early orb of venom and point booster give Spectre great early presence, also late game.
 * Skull Basher adds some damage and HP along with BKB-penetrating anticarry and chase potential that again forces attention onto her.
 * Pipe of Insight makes Spectre incredibly tanky against magic damage when combined with dispersion (combined 57% resistance), and increases her teamfight presence with the HP regen and active ability.
 * Armlet of Mordiggian has incredible synergy with Spectre's skillset. The HP and offensive bonuses work with dispersion and desolate respectively, which also transfer to her haunt illusions. The passive regen fills the need of a vanguard for her core, while armlet toggling provides increased HP for Spectre to disperse.
 * Assault Cuirass adds extra durability and makes haunt a global -5 armor for the duration while giving any teammates in range extra armor and attack speed. The attack speed has good synergy with desolate, but does not transfer to illusions.

Trivia

 * The name Mercurial most likely comes from the DeviantART account of Kenneth Tan, the person who made the 6.28-6.32 loadscreen.
 * "Mercurial" means "swift, shrewd, and deceitful", which is attributed to the Roman god of thieves, Mercury, as Spectre is an agile and quick Hero. It can also refer to the planet Mercury, since Spectre is an otherworldly entity. Mercurial can also hint towards the element Mercury, often called "quicksilver," which is another reference to Spectre's speed and mobility.
 * Spectre's alternate/fun name "Qwerty-" was given to spectre for remembering a Legendary Dota player Zaven "Qwerty" Terminasov, who died on December 5, 2010.
 * Spectre's voice responses are overlayed with the same track playing backwards, bearing a huge similarity to the Avatar of Vengeance's voice in Warcraft III: Frozen Throne, which was also Spectre's hero model in DotA.

Old Abilities
The only thing I really know is that Spectre had them in 6.30 (and playdota's changelog don't go below 6.48).