For someone not very acquainted with the game, Mercurial the Spectre can prove to be surprisingly powerful. Going through her skillset and stats doesn’t really reveal much of her capabilities, as no huge criticals or permabashes or 5 gazillion AoE cleaves are included; but seeing her in action again and again reveals her tier-1 Hero potential.
As far as late-game carries go, perhaps she’s the one that’s had the most ups and downs of all times. Spectre hasn’t really been through too many “regural Hero” stages; usually, she’s either top pick material or completely forgotten.
To understand why this happens, and it happens often, we’ll go through her playstyle, merits and flaws real quick.
As a classic late-game carry, she’s known for her relatively weak laning stage. I say relatively because she can work really well against most solo offlaners because of her Desolate. Unless it’s something like Dark Seer or Broodmother, it’s a bad idea to not go a dual or triple lane against her.
Also, her farm isn’t the most promising in the game. She got a decent boost with Desolate working on lone creeps, which helps mostly with jungling (kill the small neutrals first, so the larger ones get Desolated for more efficiency).
Other than that, good survivability and global map presence make for a decent post-laning stage.
Late game, she’s generally a beast. Haunt with Desolate and probable Radiance+Diffusal deals a LOT of damage, but what makes her so hard to deal with is her positioning. Normal Heroes have to worry about when to engage, not being too far or too close to the enemy, maybe ditch farm to join a smoke gank or a fight away from their base, etc. Spectre is not one of them. She can farm to the last second, wait until the enemy blows all their AoE, and then join the fight with her instant global teleport, while not missing any DPS, since her Haunt illusions do the job for her. In 1v1 cases, a Manta Style can destroy a lot of ambitious carries, and Dispersion makes focusing her down a double-edged sword.
Spectre is made to give her opponents impossible choices. When she Haunts, stacking together means that you evade Desolate but are vulnerable to her team’s initiation. Splitting up means tons of pure damage, too much for the poor support to even consider re-entering the fight. Pushing as 5 to end the game quickly means she gets a lane for free farm, and the ability to join her allies in defending. Not pushing as 5 means you’re delaying the game, which is usually bad against her (unless you’re constantly killing her). Leaving the game may benefit your mental health, but then you get an abandon.
Some of the previous patches favoured active safelaners and flash-farmers that could clear stacks quickly. Spectre was really too slow to catch up against all that. Going a bit further back, her core item, Radiance, had also remained irrelevant to the whole game, as it was expensive, hard to build and didn’t even synergy with other items directly.
Generally, heavy push or farm metas don’t favour her much. Spectre is best when she’s able to survive early on, while grabbing some kills with Haunt and farming slow and steady in the meantime.
The new Hero kill bounty system can grant large amounts of gold in small amounts of time, and Spectre makes sure she’s there, always. Radiance applying a miss chance means it can actually scale well into late game. Octarine Core + Refresher synergies extremely well with her buildup, not saying there aren’t other options.
Generally, I feel she’s a bit more meta-dependant that most Heroes. She has some very real and exploitable weaknesses, even after all the direct and indirect buffs. If the meta favours abusing those weaknesses, she can be countered easily, and she falls off. This is also the reason why professional teams don’t count on her too much, despite her current huge winrate in pubs; a good, coordinated lane dominance can shut her down for good.
5-man pushing deathballs are also a bane of hers, since the ability to instantly teleport in the fight doesn’t necessarily mean you can do much to stop the onslaught.
As far as Heroes go, look for those who can make illusions of Spectre and turn her own powers against her. Morphling is a good choice, but personally I like Shadow Demon best, as his full combo can actually shut down a farmed Spectre completely.