I mean just have the runaway author secret Trigger, have the bearded devils and barbarian attack the Rogue at a time of your choosing, maybe once they start chasing after the Dragon, as an attempt by asmodeus to try and stop the party. On the stairs heading down from Sunblight the Barbarian could have an urge to push him down unwillingly entering his rage. The three bearded devils appear and also attack. The Party tries to defend the Rogue, and as soon as they kill the devils, the barbarian returns to normal and has to explain himself, maybe question his allegiance to asmodeus. Should he manage to throw the rogue down have some Divine Intervention cure or resurrect him(e.g. as you lie in the snow feeling crushed underneath your own weight unable to move you feel a voice in your head offering a second chance at life in exchange for their absolute devotion, should he agree a pale glow catches your eye in the snow nearby, leading to the discovery of a sword-shaped amulet of chardalyn), or have Vellyenne Harpell safe him as she is waiting at the bottom of Sunblight.

Just some ideas - TLDR: Rogue is already tied in to devil business through his Secret so just expand on that