I was working on a doodle with some info but it’s probably easier to answer like this. :D

So, Damien is an Outer God of sorts? He’s incredibly young and markedly less powerful than most others, and might be closer to a Great Old One, but in essence, he’s an eldritch horror!!

While he comes from another dimension, he found earths cultures and atmosphere to be so engaging that he’s ended up staying a while. An interest in Christianity and the unshakable reach it has on such a large portion of the population was a particular thing he found interesting, and was what led to him becoming a priest.

At present, he lives beneath the church he practices at in order to learn all he can, even if his presence is.. starting to drive parishioners a little mad.

The thing with his legs is that his normal form is largely covered in tentacles, and he finds it difficult to keep his entire body shifted into one form for long. If his concentration slips, his legs are the first to start to lose their grip on humanity.

tldr: lovecraftian horror thinks Christianity is fucked up and cool so he’s a priest now.