A pretty big lump on old metal it required quite a lot of pinning and filling. Paint job is mostly contrasts with regular metals and some gray and white for highlights. Skin is thinned Shyish Purple, Yellow parts are Iyanden Yellow and Black Templar for used for leather.
Note the face desing on the head piece.
As mentioned daemons can be added to a genestealer cult army as summoned creatures. Greater daemons are among the most powerful creatures in the old WH40k so using them would propably be justifiable in a narrative scenario. Perhaps inquisition taking on a cult or something like that.
So indeed - the power pehind the Coven.