RWBY is owned by Rooster Teeth and was created by Monty Oum.
Five Nights At Freddy's is Owned by Scott Cawthon.
This story is a interesting project that came to mind. The story will stick close to lore for both franchises. Examples include character relations, history's, and objects/things. Yes the books matter.
RWBY side:
Ruby is Taiyang and Summers Daughter.(Sorce: Monty Oums response on the matter)
Neopolitan is not related to Roman. They have a semi romantic veiw of one another as far as I can tell. (Sorce: RWBY Roman Holiday)
Coco is a lesbian. (Sorce: RWBY After The Fall/Before The Dawn)
FNAF side:
William Afton is obsessed with Remnant(soul juice)
Springlocks
Elizabeth Afton's obsession(runs in the family I guess) with pleasing her father.
Things like those and many others will be present and for Unconfirmed Pieces of lore specifically for FNAF I'll be ignoring them or reference them in a minor manner. With FNAF characters showing up in Remnant(the world) bits of past events will be different specifically for the Rose/Xiao Long household.
With all of that said I hope you all enjoy.
-Never The Less
