Quote Originally Posted by Grendelkin View Post
Prediction: Xykon's tomb/fortress on the Astral plane will be completely annihilated. When he realizes this has not harmed him at all, the lich will realize Redcloak pulled a fast one on him with the phylactery. It is then that their alliance will end and they finally come to blows - and the Monster in the Dark will be compelled to devour Redcloak.
In 3.5, liches can survive having their phylactery destroyed, no problem, as long as they're not currently using it to regenerate. This was brought up in Start of Darkness.

Way back in 1E/2E AD&D, destroying the phylactery would instantly kill the lich, but that's not how it works in the OotS-verse. I'm not sure if Xykon would even know if his phylactery was destroyed if it was done out of his sight.