Grulid is in the rebel aqueducts at location 1500, -45, 143 (also findable with CTRL+F). To get to the aqueducts, you can jump down the well in the very far northeastern part of the zone or use one of the four portals scattered throughout the zone. (For mapped locations to the aqueducts, see this image.)
For all things Rain of Fear, check out this overview of the expansion.