Unlockable Secret Characters
Take control of Kislev’s three major cities: Kislev, Praag, and Erengrad, to get the Frozen Falls quest battle that unlocks Boris. River Urskoy, The Cursed City, and River Lynsk each have their own province capitals. The quest battle marker appears just north of Praag after you hold these settlements for ten turns.
Katarin’s campaign is your best bet if you want to unlock Boris Ursus as soon as possible because she starts in control of Kislev, is close to Praag, and is friendly with the faction that controls Erengrad, meaning a confederation is possible if you play your cards right. This should allow you to unlock Boris fairly quickly.
To unlock Be’lakor, first, play through the Realm of Chaos as any faction – collect the souls of four Daemon Prince Champions, find a way to the Forge of Souls, and fight Be’lakor in the final climactic Survival Battle. After defeating Be’lakor and his allies, the campaign will come to a close with a few cutscenes, and the player will be given the option of ending the campaign or continuing. Tzeentch, the most cunning of the Chaos Gods, offers Be’lakor their physical body back, but with a catch: Be’lakor must now serve the faction that defeated them in combat.