Arazni's almost too obvious to be it. Especially since she's somehow not mentioned as an in-setting theory, even though supposedly her old pre-Geb teachings were still followed by a rare few in Lastwall.
My bet is : no single entity, but a small alliance of deific powers.
The Ragathiel hints are strong as well, arguments could somewhat be made for a few others and really, what kind of do-gooder wouldn't oppose goold old TarB'.
|The Drunken Dragon|
I mean given Arazni's aesthetic it seems a reasonable choice. Maybe Nocticula, but that seems far-fetched given that her color palette doesn't match and she has no real relation to the undead. It seems odd why a being like Ragathiel or Milani (another guess of mine) wouldn't just...claim responsibility.
Also Arazni feels more appropriate given the oath, so...
|The Gold Sovereign|