How do you get boss challenge rooms?

[Asylum]

i got it on one of my files before i overwrote it, and now i cant get the challenge room after i beat the boss *on ng+ on my new file still cant get them*

[johnsoma]

There are special talismans connected to the bosses you can get from the archangel rooms after beating the normal versions. Once you get one, kill yourself and give that tailisman to the gatekeeper and go to the area where that boss was before. You'll fight a much stronger version of that boss with a predetermined character.

If you're on ng+, I'm pretty sure you'll have to beat them again in order to get their tailsmans, but I'm not sure about that.

[Asterio]

Originally posted by TAW johnsoma:

If you're on ng+, I'm pretty sure you'll have to beat them again in order to get their tailsmans, but I'm not sure about that.

Yes, he will have to.

[Defiled_Popsicle]

To do the neo bosses you need to get special "Obols" from the prayer statues. Charon will take them and you will unlock the optional bosses.