Magma Rivers is a unique Lil' Darksider miniboss encountered in Triton Flats in Borderlands: The Pre-Sequel. He is found only in the central building in Triton Flats, which houses a Catch-A-Ride station and the level's Fast Travel, on the bottom floor .
Magma Rivers is a Lil' Darksider miniboss, and has increased health and damage from the base enemy. He will also drop better loot than most Darksiders tend to, though doesn't have any unique loot associated with him.
During the mission A New Direction, killing Magma Rivers will cause him to drop a prism, which is a required objective to complete that mission. Afterwards, Magma Rivers will continue to spawn in the same area.