Kostaltyn
The Supreme Patriarch of The Great Orthodoxy, Kostaltyn is an original Legendary Lord for Kislev in Total War: Warhammer 3. Being the child between Creative Assembly and Andy Hall. He pretty obviously looks like Rasputin.
The Great Orthodoxy itself was created by Tzar Boris Bohka, who wanted to codify and centralize the religion of his country ala Henry the 8th. Unlike dear old Henry, however, Boris had a very good reason to do this as many cults usually worshipped in the woods through strange rituals that he was scared would turn them to Chaos. As such, he created The Great Orthodoxy to govern the worship of Kislivite religion and and see to strict oversight.
This presumably moderate system meant to help a secular nation got a whole lot more radical after Boris disappeared and Kostaltyn took over. Many worshippers of Kislev's religions are split between holding on to their old ways and sticking to this new structured format, obviously Kostaltyn supports the latter. Part of his resentment of Katarin is he views her as a supporter of the old ways of worship, and her being a mage doesn't help (even though Ice Magic doesn't actually connect to the warp in anyway, but I doubt he would know or care to learn that.)
Similar to the other Total Warhammer OC, Cylostra Direfin, the reception of this man was... mixed. Some say that he is an excellent addition to help flesh out the rather small collection of named characters Kislev has and has an awesome design. Other say that he was completely unnecessary and that he stole Tzar Boris's spot as the second Kislev Legendary Lord. No, this disagreement probably won't end for a very long time.