This. The implicit assumption of several lines of Redcloak's dialogue is that he already tried everything he knows how to do. I just didn't show it all because I had limited space, and because I assumed that the reader would understand that if a smart, tactically-minded caster like Redcloak says, "X is the only way out," then he probably tried the easy ways first.
After all, Lirian would have needed to ward her prison against people coming in to rescue them, too, which means locking it down to plane shifts and stone shapes, even if she thinks her prisoners are incapable of them. A restored Xykon, however, is more powerful than Lirian could ward against.
That stuff cannot be penetrated by a high CL. how did Xykon break out. I find your information makes it hard to believe he meteor swarmed out and he wasn't epic back then
We're all in the Monster Manual somewhere. My entry is between Howler and Hydra.