Well, basically, it COULD be like this:
Hazama experimenting with cauldron -> Black Beast accidentally created on the spot (not Ragna and Nu) -> defeated by original 6 heroes -> start time loop -> BB spawns when Ragna/Nu sent back in time -> loop til it's broken
Or it could be like this:
Hazama experimenting with cauldron -> Black Beast, which is Ragna/Nu all along, comes in from the future, which is what Hazama calls an accident -> cue time loop