Crystalline Cleric Beast - CR 11 bossfight for D&D 5e

Thought I share with you my freshly playtested/homebrewed bossfight :D 4 adventurers on level 9 went against him, and he turned out to be quite a challenge! If you do use him in your games, please tell me how was he received :D (Illustration was made by me, but it's still WIP - painted on paper with White nights watercolour and Windsor-Newton Indian ink)

If you're DM-ing him, be aggressive, and don't forget to use his lair/legendary actions. As for the Crystalline Bloodcurse, it's up to you DM what will be the effect ;)

Backstory:

One of the higher priests serving the Brightfolk (a faith worshipping the Sun) was a bit more brave/stupid than normal, and out of pure hunger for power he delved into the art of blood magic. (which is prohibited both by the country's law and the "magic police")

He sacrificed countless people capable of magic, and turned their bloodborne mana into raw power. However, blood magic is banned for a reason, and that bloody mana started to grow out of him from the inside out - transforming him into this amalgamation of crystal and flesh. He was locked away by his church, but a mysterious somebody released him from his confines and brought a bloodborne plague upon the city he was prisoned under.