According to the Binding of Isaac Rebirth wiki:
If The Book of Revelations is used at least once in a map, a Harbinger will replace the normal level boss.
I've found Book of Revelations in a Library Room in Basement 1 after defeating Pin boss. So I moved to Basement 2 in order to activate the book and make my next boss Famine. In theory.
The fact is that I found Larry as Basement 2 boss. How is this possible? I mean, the book should replace my next boss with a Harbinger. Why not this time?
Meat Boy and Bandage Girl achievements are driving me nuts…
Best Answer
I've heard somewhere that the Horsemen can only spawn on the first floors. Basement/Cellar I, Caves/Catacombs I, Depths/Necropolis I, etc. But I can't find reliable information online to support this.
Also, this might also be why - According to platinumgod.co.uk,
This might mean that the Book of Revelations only gives you a higher chance (although a very high one at that) of spawning a Horseman for the boss.
And if you're looking to get the Meat Boy/Bandage Girl achievements, save yourself the pain of Conquest ruining your day and go for the Monster Manual + infinite item charge approach.