it's an unbreakable contract known which makes the deal a prophecy and a law of the natural world. after Banner signs it to hand over Hulk's body in exchange for the survival of their companion, Charlie, the Eldest child of the Mother of Horrors binds Hulk in the unbreakable chains of eternity, made from the
First Firmament itself, and is about to possess his body, which was granted to her according to the pact, but it doesn't work and she's blown back into a wall. in shock, she repeats that the pact is unbreakable and asks what protects Hulk, which to me implies TOBA since TOBA broke a previous incarnation of Hulk from Eldest's possession in the past a few issues prior. and after this, Hulk just breaks his way out of the chains too