I'm playing a three-player FFA game, and one person is playing The Ice Court. He completed the quest battle for Boris and chose not to give him a city, which we understood to mean that the game would add Boris directly to The Ice Court as an additional Legendary Lord for him to use during the same campaign despite the tooltip saying he disappears (but if the option is Join Your Faction and it actually does mean he goes away forever, jeez, please change that to not say "Join Your Faction"). Boris never spawned, however, on his list of recruitable lords, the list of active lords, as a wounded character, or anywhere on the map in general.
If it matters, our other two players are the Poxmakers and the Northern Provinces, and he completed the quest with a generic Ice Witch Lord because that army was stronger than Katarin's at the time.
We did see that the Ursun Revivalists are available for multiplayer campaigns right away, does that mean his faction existed on the campaign map already and that he couldn't exist twice? None of us came across him, but we didn't start close to Zorn Uzkul and I never saw him with my caravans.