After Ring of Power, you'll need to dominate a few enemy captains to fill out ranks. Every time you do so, Bruz will show up with some relevant tips on how to turn a captured captain into a bodyguard, or a quisling that can backstab an enemy warchief.
Once you've put together a sizable enough force, this mission to attack the Nurnen fortress becomes available. Bruz is automatically assigned to your army- you can add other recruited captains to your ranks, and spend mirian to upgrade their offensive capacity, if you wish.
Once you've made your selection, the battle is joined. Minor spoiler: this particular fortress doesn't have an overlord fight, so don't be afraid to use your resources for the siege objectives here.
Your goal is to capture the two Victory Points in the castle, but once you're in place, you priority should be taking down (or recruiting) enemy captains. They're the biggest threat to your own forces, and they have a knack for ambushing you from behind when you're working on something else.
When you've held a point long enough, you'll get a prompt to claim it with the  key. Do so to capture it permanently.
Once the outer fortress is yours, follow your objective marker to the Keep for the final confrontation.
While you'd normally be facing an Overlord fight here, in this instance you'll be treated to a lengthy cutscene, and a reunion with an old acquaintance from the previous game.
After this scene plays, you'll go outside, speechify to the troops, and the fortress is yours.
The region is yours as well. This technically completes the mission.
Still, conquest isn't all fun and games- you'll have to pick a new overlord and shufle your troops into desired positions before you can move on. None of these choices are permanent, so go with your instincts, then move on.