Vampereon by @Eternity9

Type: Dark / Fairy

Ability: Cute Charm

Hidden Ability: Dark Aura

Classification: Vampire Fakémon

Dex Entry:

           Vampereon the rare, but deadly Eeveelution it's existence is considered a Myth, but stories of the horrors committed by this fakemon are wide spread. Its unfortunate victims that fell for it's Cute Charm would be found later drained to a lifeless hush, it is also said that the only way to get this Eeveelution is for someone with a heart of pure evil were to raise an Eevee. 

I actually like the idea that this Fakemon is actually a really loving Fakémon despite what the myths and stories may say about it XD

Game mechanic wise:

I imagine again that this would be an event only Pokemon, the player character winning a 'Vacation' to a mysterious resort (That's actually a spooky castle), and the player character traversing the castle grounds getting past the puzzles, maze garden and the other 'guests', eventually leading up to a battle in a crypt against an ancient Noivern (Totem Pokemon Battle) after the battle you are able to obtain the item 'Dark Fang' and give it to Eevee to evolve into Vampereon