Myrkul is a Quasi-Deity at this point, same with Bane and Bhaal. They have fallen so far from thier former heights - they must be desperate and they are weak (relatively).
Plus there’s Cyric. I don’t know if it’s possible he’s involved. Looking at the FR wiki, it seems he’s still probably imprisoned as of the BG3 era, but at least has manifested as a vision in the years beforehand and presumably is keen to escape.
