Rift Quest:In Corruption's Shadow  

Note that there is a Healer standing next to Borrin.

Each Volcanic Vent you seal will reveal an Ashen Cindergut. The vent respawns fast.

From the tunnel and facing Borrin, go left and up a barely-noticible incline to the ledge with a magma vent, on a promontory, spewing lava and surrounded by Magma Spouts. The Spouts are easy to single pull,and after killing six or so the Magma Lord will appear.