Destroy the Gravemind and everything falls apart.
Look up the Gravemind. It will tell you what you need to know.
The Gravemind is a giant Tentacled biomass that has the knowledge of every single being absorbed.
No magic or abilities of the absorbed, only memories and whatever was known by the absorbed being.
If your second in command gets absorbed, then the Gravemind knows everything it knew. Once absorbed, there is no coming back!