Just waiting for my Knowledge check results (or lack thereof), in case I get something like "they have personal Antimagic Fields" or "they have personal Anticipate Teleport fields", or something saying "You recognize the man as an extremely powerful demon, a much bigger threat than the brutes". Assuming nothing like that, though, yeah, I'll teleport Derk in, aiming him at one of the brutes.
If the one Rhion went for is still alive, I'll target him, else I'll go for one of the two further back. In either case I'll move to a 5ft step away from them, float up off my Phantom Steed (5ft step with Perfect flight), and Benign-Transposition myself with Derk (who I should be able to sense through blindsense, I hope). He's flying iirc, so this should avoid any potential confusion with what happens when you BT a mounted character. If there's a line-of-sight/effect problem with this plan, let me know - in that case I'll probably pull Valthrax out again and get her to fly over, then transpose her with Derk (since Umbriel will have line of sight/effect to both of them), or just use a different spell.
Togo, if this gives you enough information to resolve my actions, please feel free to continue, I can edit in the flavor text later.