What the hell does a pregency of a Balor, and a Fallen Celestial produce anyways? Would be really helpful to know, Thank you for you time in even reading this.
I consider these kinds of outsiders to be sterile, except when with mortals (demons are not birthed by sex, but mutated from the tortured souls of the wicked).
You can go horrific (any of the ELH abominations), prophetic (axiomatic human), biblical (a set of twins - one solar one balor), perhaps the munchkiny (a half-celestial balor or half-fiend solar), the comedic (a kobold?)...
This seems like something that's campaign-bending, if not breaking.