An Elden Ring player has discovered a new bug in Shadow of the Erdtree that renders Promised Consort Radahn completely helpless against your attacks.
As seen below, one Elden Ring player was journeying through the Lands of Shadow with their friend in jolly cooperation when they came face to face with Shadow of the Erdtree's final boss. The trouble is, Promised Consort Radahn wasn't reacting to their attacks whatsoever; instead, he was just standing there menacingly with his two weapons drawn.
Did I break my game? from r/Eldenring
It's tricky to work out what's happened here, but from the video footage above, Ansbach literally hands the player the Remembrance of the final boss, before disappearing. It's after this that Promised Consort Radahn is effectively rendered motionless, implying that the bug is somehow linked to Ansbach or his overarching quest in Shadow of the Erdtree.
According to the comments, this is a new bug that's only just come into play with Elden Ring patch 1.13 earlier this week. Allegedly, the bug can be activated if you complete the invasion encounter in Enir-Ilim's antechamber with a co-op partner. One commenter claims to have activated the bug "every time" they completed the antechamber encounter in this way.
Ansbach handing out Promised Consort Radahn's Remembrance possibly implies he has to be used in the encounter against Needle Knight Leda and her allies for the bug to take effect. That's entirely unconfirmed and pure speculation right now, however.
Right now, the bug has been recorded on PC and Xbox platforms, but there's no mention of anyone having seen Promised Consort Radahn rendered helpless on PlayStation platforms. If I were you, though, I'd still steer clear of attempting to complete the antechamber invasion with a co-op partner on PlayStation platforms, for now at least.