Swallow

Swallow: If a hero is defeated by this monster, remove his hero token from the map and place it on this monster’s base. The hero cannot be revived until this monster is defeated, at which point his hero token is placed in one of this monster’s spaces.

FAQ
Q: Does the Swallow ability prevent standing up, or only revival?

A: The Ice Wyrm Swallow ability only prevents revival, not a hero performing a stand up action. (source)