The first step is to complete the entire Bruz
quest line and unlock Skill Upgrade: Worse Than Death
. Once unlocked, equip it.
Next, you'll need to find an enemy assassin in the army menu. You can use a worm to find intel on captains, or shun an assassin follower to turn them against you. You can even use loot box cards to summon the follower you need if necessary. You can see their whether they are an assassin underneath their name or in their details under Class Traits
and Advanced Class
With the skill equipped and an enemy assassin, you will need to find them and dominate them, and then use the Shame - Worse Than Death
option until they become deranged. There is a chance that this skill will actually raise their level instead, but just keep trying until it works; it may take multiple attempts.