Guys, just imagine it!
Sora & co. stop by a strange new world where a lot of children have gone missing. They decide to investigate the rooms of the kids and find many shadows, the remnants of the obviously heartless culprits. After defeating a few, a certain blue-green beast pops out and yells, as if he were somehow trained to scare any being in existence. The remaining heartless flee, not so much out of fear, but out of pure instinct to run away (shadows being known to phase into the ground and leave an area during cutscenes especially).
The beast, calling himself Sully, has also noticed that kids have been going missing. Needing the screams/laughter of children to power his world, Sully joins Sora's party to help find and save the kids.
Oh yeah and Mike Wasowski was somehow involved.