Canon: Undertale
Villain/Antag/Monster: Omega Flowey
Type: Boss
Description: Wiki link. Basically, a Flowey before character development, powered up by six human SOULs and transformed.
Notes/Adjustments: For everyone that's actually done the Omega Flowey bossfight, I don't think he'll need too much adjustment besides being able to be killed.
Will it exist after the event? NO
Questions/Other: I do have a journal set up for this battle if need be