Entered boss realm, beat 3 bosses, ended up in a big cave with a large tree and a stone column, but no boss to fight. Had to fast travel back to respite and reset the boss realm. Tried again with a reset and new card, this time I only beat 2 bosses before ending up in an empty cave with no boss to fight.
There is a small gap in the floor close to the edge of the wall. Follow the wall around the cave and you'll eventually hit it.