This guide will let you know how to unlock Boris in Warhammer 3. Go through the guide, and follow the step-by-step instructions to unlock it.
First, you need to have some main Kislev settlements, which can be done with a motherland mechanic, and it will allow you to confederate your political opponent. If you are playing as Keterin, you can easily get two of these settlements, as shown in the image below.
There are three different locations for each settlement, and you can find the first one to the west of Kislev land called Erengrad. On the right side of this settlement, you will find the next settlement called Kislev, and the last location is on the north side of Kislev called Prague. Once you have these three cities, you have to hold on to them for ten turns, as shown in the image below.
Once you have completed it, you will find a quest battle Frozen Fours which will allow you to save Boris and unlock his campaign. You can also use him in custom and multiplayer battles, as shown in the image below.