Rumored to be the shadow of Clefable, Gengar has always been an amazing Pokémon and it pretty much fulfills the same role as it did beginning in the fourth generation: a deadly threat with Shadow Ball, Sludge Bomb, and "Focus Miss"/Focus Blast and various options to mess your opponents, such as Will-o-Wisp, Destiny Bond, and many more options. The devs have also blessed Gengar with the illustrious Nasty Plot, the move that Gengar should have in its arsenal due to its appearance, to further its Special sweeping capabilities. The downside is that Gengar is pretty frail to utilize Nasty Plot to its fullest extent. The lack of Life Orb and Choice Specs unable Gengar to KO opponents reliably. Also, Ghost/Poison offensively is not that great due to the fact that Fairy does not exist and the fact that Steel did not lose the resistance to Ghost in this game.
Overall, Gengar is a very powerful Mon to use against the right opponents.