Tran's spellcasters, exhausted by design under constant trebuchet fire and committed to the southern assault.
The crawler army's mages are Tran's command, and Boomer refers to them as "your magic slingers" when he asks whether they are ready for the assault. Tran only nods, his metallic crab legs telegraphing how nervous he is.
They are the enemy's real target in the bombardment. The pounding from the Artillery Batteries is constant and specifically designed to exhaust the crawlers' mages, meaning the mages' fatigue is a resource the enemy has been draining for as long as the shelling has gone on — a hidden cost the southern assault has to pay before it starts.
The crawler army's spellcasting arm, supporting the southern infantry assault.