As simple as it sounds, there is a reason no one has written a guide for this. But here we go:
The King Bible is a weapon you can obtain while leveling up. It is also the starting weapon of Dommario, who you unlock by getting 5,000 gold in a single run. It is a book with a white cross on a black ground ground and blue trim. It will have a chance to show up each level as long as you have weapon slots open.
Once obtained then you just have to select it’s upgraded upon level up until you reach level 4. To go the extra mile here is what each level entails:
Level 1
"Orbits around the character."
Level 2
Fires 1 more projectile.
Level 3
Base Area up by 25%. Base Speed up by 30%.
Level 4
Effect lasts 0.5 seconds longer. Base Damage up by 10.
Done and done