Kostaltyn

This article contains spoilers! You have been warned.

"Ko-Ko Kostaltyn
Doesn't like the Kislev queen
There was a cat that really was gone
Ko-Ko Kostaltyn
Kislev's greatest love machine
It was a shame how he carried on."

– Kislevite song

So badass his mace is a burning church.

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. Amusingly enough, despite being made to ressemble the real slavic "konstantyn" name, Kostaltyn's name is a mangled mess almost unpronounceable by any actual Russian.

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 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. Or at least until Boris was also revealed to be playable at launch.

Rediscovering the Red Tsar

While it isn't exclusive to Kostaltyn's campaign, once the player has unlocked the requirements (holding all 3 major cities of Kislev for 10 straight turns), a cutscene plays where the patriarch rediscovers Boris magically frozen in ice. The latter soon frees himself, at which Boris becomes playable for the campaign.

Hilariously, despite being implicit in freeing Boris and being the head honcho of the Orthodoxy, he does not get any bonuses to Diplomacy with Boris' subfaction.

In Game Abilities

Kostaltyn is a melee-focused lord whose abilities increase the damage Kossars, Kossavite Dervishes, and horse archer units, and gives other Patriarchs an increased ward save. He has access to Battle Prayers as well.

His unique trait is Supreme Patriarch, which gives him quicker personal Wound recovery, and Frenzy to his entire army.