Thanks to @ for suggesting this! I actually had a lot of fun making this Mega.

Darkrai would keep its ability after Mega evolving, bad dreams, which does damage to your opponent when they're sleeping. Darkrai gains the ghost type, making its only weakness fairy and giving shadow ball and phantom force STAB. It gets a powerful boost to its sp attack and speed and a slight boost to its attack and sp defense and a a drop to its defense. Its appearance changes to where it now has a grim reaper style sickle as well as shadows seem to be trailing from its back. It's bigger, scarier, and more intimidating and grim reaper like.
Stat boosts:
45 sp attack
35 speed
- 20 defense
20 attack
20 sp defense