PS: Girgashiy do not come from the other world. They're created from Erinn's humans using the divine power from the other world.
I know they come to being through corrupting humans, the last scenes in G19 already told me that. But prior to that, Avelin had said before the power (or existence?) of their god (so by extension, them) came from beyond the 'doorway' we let Ruairi open back in Saga II, thus why the Milletian was under suspicion by the Alban Knights to start.
They're definitely not native to Erinn, that much I understand. I don't really understand any further than that.