The Black Bishop is a secret boss you can fight on floor 12, after killing the White King. He's quite strong so make sure you have a good build going. This guide will show you how to summon him, and get the Avenged and Exorcised achievements if you manage to defeat him.




I would recommend playing on easy or normal difficulty, and you can't get both achievements on the same run, more on that later.
Since the new update few things have changed, mainly:
-Secret Heir is now theocracy compatible.
-Theocracy and throne room now exclude each other.
-Exorcised steam achievement doesn't block the unlocking of Avenged steam achievement anymore.

You will need two White modifiers in order to get the encounter:
-Theocracy
-The Red Book











Getting the Red Book is easy, it shows up quite common so there is nothing really else to add to it.
If you're struggling to get Theocracy that might be because there are certain restrictions on cards when they pop up at the end of a floor. Theocracy seems to have a few:

-White needs to have at least 2 bishops.
-You must not pick Throne Room and Ritual Dagger cards.

The White King fight is as normal as it is with one exception: before killing the boss, leave exactly one Bishop alive (the amount of other pieces don't matter).

My advice is to keep distance from the king while you clear other pieces. The Moat is a good buff which gives you more time.

If you are successful the bishop transforms.
Instead of trying to chase you the Black Bishop summons pieces to fight for him, and periodically heals them. It may seem better to finish him fast but be aware that things can get out of hand quickly so keep some shells for the minions.

IMPORTANT: You have to kill the Bishop with at least 1 shell remaining in your shotgun (meaning the moment you would kill the Bishop you have to have 2 or more shells loaded like in the screenshot). That's why it's important to play on lower difficulty, so you start with 2 chambers that way you get both achievements.

If you succeed you will get the achievement and a secret ending.

Thanks to everyone's feedback in the comments I improved the guide and provided more information which I wouldn't have been able to otherwise:
-Ibottas and Altivu giving information about the other ending.
-Eno figuring out that reinforcements don't have to be killed against the white king.
-Alan who did some digging and provided much appreciated data regarding the theocracy card being really rare to show up.

Thanks again, and don't forget to play old school chess from time to time!