Belub can be first encountered in Moonrise Towers' main hall, observing a group of new Absolutist pilgrims. He shows contempt for the pilgrims, and remarks on their lack of skills and usefulness. The party can then either convince Belub to keep the pilgrims, or tell him to get rid of them.
[PERSUASION] They have potential. Great warriors have grown from less. (DC 14)
Its not our place to question the Absolute's will.
Then get rid of them. The undeserving should be purged.
The first two options let Belub send them to the war camp west of Reithwin, where the pilgrims then disappear. Eventually telling Belub to get rid of the pilgrims results in the death of Pilgrim Farimorn, Pilgrim Iong, Pilgrim Nalira, and Pilgrim Taima.
If the Nightsong is freed and the Harpers launch their assault on Moonrise Towers, Belub is part of the remaining forces defending Moonrise Towers. Belub can be found on the rafters above the ground floor's main hall, where he can harass the party and their allies.