Cleric of Roavgug - humanoid "spawn" build

Hey everyone, here's a little challenge for you.

So I have a cleric of Rovagug who sets out to awaken one of the spawns of Roavgug (Volnagur). My idea is that this attempt backfires and the cleric turns into a much weaker, humanoid version of the spawn, or simply adopts some of the spawn's characteristics. This character should be a major antagonist for the PC's. I am looking for a CR 12 build.

Any ideas how this can be done and where to start? How should he look? What special abilities should he have? Just weaker versions of the spawn's abilities? What class should he be under? Should he still have hos cleric spells or shall I transform him completely?

Any and all thoughts are welcome.

Thanks you!

Tossing on the Kanabo template is similar to weakened versions of the powers of a Spawn, and so is the Apostle Kyton one if you did some light tweaking. They can serve as a base to calibrate the power of this character.

How about a Rough Rampager archetype Antipaladin instead of Cleric? Seems more straightforward to build, and can make better use of the spell-like abilities granted by those templates.

I found the Rampager template but not the Rough Rampager. Do you have a link by any chance, or can just tell me what the difference is?

The reason I'm inclined to leave him as a cleric is because it's part of his background story and I already wove this into the campaign as he is the BBEG, but I am willing to consider the Antipaladin idea just for the sake of stat blocks that best mirror the idea of a "humanoid spawn", since this is what he's turning into towards the end. I mean, I will prefer the build that can give me the right flavor but without too much complexity.

Thanks!

Here ya go.