On second thoughts…

They’re the anchor of the Free Mantle Systems

Maybe my Valhallans were fighting the anchor of the Free Mantle Systems, the Freo Marines?

Freo! Name: Freo Marines
Founding Chapter: The WAFL Marines
Founding: Unknown
Chapter Master: Matheus Pavlach
Homeworld: Freo
Fortress-Monastery: The Dock
Battle Cry: Freo! Way to Go!

😀