Devil deal item as reward for clearing boss room?

[KurisuSF]

So i got this weird reward when clearing a boss room in Downpour I, the boss was Rainmaker. Anyone know why this happened? None of my items should have that effect and I've defeated Rainmaker before and this never happened. https://imgur.com/gallery/OCF4gRO

[guts' theme (Cut & Looped)]

Judging by the library/book room you probably used the Satanic Bible which alters the boss fight reward to be a devil room item for the same heart cost as a devil deal.

[Zeforas]

Did you use the satanic bible on this floor?

[KurisuSF]

Oprindeligt skrevet af guts' theme (Cut & Looped):

Judging by the library/book room you probably used the Satanic Bible which alters the boss fight reward to be a devil room item for the same heart cost as a devil deal.

Oprindeligt skrevet af Zeforas:

Did you use the satanic bible on this floor?

I did use it, so it has this effect now in Repentance. Good that I know it now, thank you both!