Soul Eaters

[Ocean]

"Dementors" should come back, renamed as "Soul Eaters" or something else. Some spell could be their weakness (much like Wisps only die to Waddiwasi); I'm not going to suggest the Patronus Charm since that too has been removed from the game, but it would be pretty awesome to see something similar to the Patronus Charm, perhaps named differently, or it could just be a spell already in the game, like Glacius or Tremorio.

[Skystone]

This sounds pretty good. I would go with Glacius being their weakness. If they're anything like ghosts, mud would go throw them. Ice on the other hand could have a chance of freezing, and then killing them.