Gourgeist the Halloween Pokémon. Like Delibird, its other Holiday themed counterpart it has a weird gimmick -in this case a multitude of sizes that shuffles its Attack, HP and Speed. And like Delibird, they are plagued by a myriad of common weaknesses and their stats remain poor, preventing them from receiving widespread play, despite the decent support movepool (and Gourgeist's actually decent pool of resistances). The true terror of Gourgeist is how bad gimmick Pokémon are, forever wandering the shadowy depths of obscurity.
Trick Room or Treat
Gourgeist (Super Size)
Ability: Insomnia/Frisk
Item: Leftovers
-Trick Room
-Leech Seed
-Trick-Or-Treat/Will-o-Wisp
-Poltergeist/Shadow Sneak
Nature: Sassy (+SDEF, -SPE)
EVs: 252 HP / 252 DEF / 4 SDEF
Trick Room Gourgeist. With its decent HP and common resistances, it can actually take a hit and start harassing the opponent with Leech Seed, Trick-Or-Treat and Will-o-Wisp, before setting up Trick Room and switching out to something that can abuse Trick Room, with Leech Seed recovering damage on the way in and Will-O-Wisp ruining Physical Attackers. Trick-Or-Treat pairs well with slow Dark type attackers, like Crunch Snorlax, Incineroar, Scrafty, giving many targets an extra weakness to break through. STAB move is to prevent Gourgeist from being Taunt bait. Poltergiest is powerful, but can be walled by anything without an item (from say, Knock Off) while Shadow Sneak is weak, but fantastic at picking up KOs on Pokémon barely hanging on.
Other Options
Destiny Bond -could be used as a surprise suicide set, have Gourgeist setup Trick Room while taking a big hit, then take advantage of its increased "speed" to throw up a Destiny Bond to take out your opponents Pokémon and get your Trick Room sweeper in free. However, both it and Trick Room are stopped by Taunt and once Trick Room goes up, people tend to switch to stall out Trick Room or to go to their own slow Pokémon to take advantage.