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