Show the cultist dispatch to the commander of the crusader.
Scout out the tower of Estrod.
Report to Irabeth about the planned assault (or the victory if you killed all cultists there.)
Other Pathfinder Wrath of the Righteous Guides:
The Final Dagger Walkthrough
Once you reached the surface, you can show Irabeth the cultist dispatch at Defender’s Heart.
Travel to Tower of Estrod, when you enter the tower the first time, some ghosts of Aasimer will appear. Depends on your alignment, they may cast Bless spell on you.
After finding the disorganization in cultist ranks, you can report to Irabeth for the quest The Final Dagger.
If you insist to assault this demon bastion, you can try to send one of your teammates to sneak to upper side of the tower and knock down the pillar, it may kill some of the cultists.
Once you clear the cultists here, you can proceed to the basement and loot a +2 Chainshirt, a +1 Longbow, and a +1 Kukri. You can also decide what to do with the custodian and worshiper.