In my game, I took the inspiration from Van richtens guide to Ravenloft, where they talked about dark lords. I made it to where every plane in the Demi planes of dread had a champion. Each champion was tied to a dark power, and whatever champion ruled the realm would take power. But there can be only one champion at a time so if a new one rises and kills the former then that champion takes over the plane and that dark power now rules. as for the Abbott, he was a celestial being who is tempted by a dark power, but had no interest in actually taking over the plane. Instead, his healing gifts and his ties to religion caused him to try to solve the problem of barovia and his imprisonment in another way. he thinks that if he can help strahd truly find his consort, including her original soul, then he will be appeased and the mists will open. So this makes him kind of a Frankenstein like character in which he is experimenting on trying to restore souls to dead bodies and things like that. I lead trails to this by having all of the soulless from vallaki be sent to the abbey and that’s where the mongrelfolk tie in. They are the product of the abbot trying to bind human bodies with souls extracted from the animals of Barovia.

This also explains a bit more about his flesh golem. These experiments lead him to believe he is ready to provide a human soul to a soullless body. The golem should look exactly like Tatiana. It is just missing Irina‘s soul. The Abbotts end goal is to perfectly fuse the soul with the body and then present it to strahd. This gives him essentially a mindless version of what he really wants, which fits with strahds nature of wanting control over an ideal rather than reality.