In short: Tarnum desperately needs to disguise himself as a Necromancer. The best way to do this, naturally, is to capture a castle, but the castle must be empty, unoccupied by anyone. Otherwise, the rumor of a Necropolis being captured by someone will reach the ears of the Necromancers, and Tarnum will have to abandon his dream of becoming one of the undead. Such a castle, abandoned by its previous owners (or perhaps cleansed of them?), is discovered to the east of his lands. He hastily builds fortifications next to it.
But the rulers of the lands and the Dungeon are not happy with this situation – and who would like a barbarian invasion? No one, and they, having previously formed an alliance, declare war on him...
Whether Tarnum will survive is unknown... :D
