Turns out I have done an ungodly large sequence break and went straight to Obsidian's Temple before EoW. And that wasn't the intended Brain of Cthulhu fighting area, the one I fought was in the east near the jungle.

There is a bloody spine in a crimson section of the Corruption Temple with hearts to fight BoC, shortly after finding the Cloud in a Bottle. There's another bloody spine in a chest at the end of the oolicile forest but that one is a bit hidden (clue: broken bridge secret)

Oh also there is a Rematch Tomb for all or most bosses:

Recipe:
DarkSoul, 25
Book, 1
at a DemonAltar