The rock minions are a type of small earth elemental that Fadeer could summon using a ritual. When they're killed, they crack open to release a pool of lava, which functions similarly to a wall of fire.
The party first encountered them in the tunnel between the Maintenance Station and the Northern Terminus, giving chase and eventually defeating them alongside some fire guys and a pair of goo people, later discovered to have been in the employ of Lady Myulana. They later form part of Fadeer and Imasa's Polaron invasion force.