The Plane of Hypnerotomachia is a different dimension, or universe, or something. If Damian or Ygerna don't have some kind of power or control there, they they have the services of someone or are in possession of something that does. This plane is not mentioned in Beyond Divinity, though that game does contain a bit of info on other realms reachable by rifts.
In Ygerna's memory you are fighting her memory of Zandalor and the Divine.
