Tam-Tara Deepcroft and Edda

[Magis]

Maybe it's Ampador-brand White Magic mixed with thamaturgy then?

You are basing this on the Baalzephons and the Allotaur (iirc?) appearing in both instances (Ruins and Tam-Tara)? The reasons the voidsent were in Ruins was because black magic put them there. Diabolos was summoned by an enemy nation of Amdapor from within, and white magic bound and sealed them inside (see the white magestones holding back the Baals). Probably along with Diabolos, some other things seeped through too.

I think Edda was just so lost in darkness that she was consumed by it and turned to black magic to get her boyfriend back. Another person on Reddit suggested she made a pact with the voidsent like the boss from Haukke Manor did. I could see it also explaining how she came back from the dead.