After defeating Ketheric Thorm or convincing him to defeat himself by persuading him to repent, you will fight his god, Myrkul. Myrkul will consume the Necromites around the room to gain additional power. When the battle starts, wipe out the group of 4 Necromites on the upper left platform. After that, watch for the creation of Skeletal Involucres which will also turn into Necromites. If you saved Nightsong, free her from the top right platform. A character with extra movement abilities can reach her in 1-2 turns and let her join the fight. Once the Necromites are dealt with, focus fire on Myrkul to earn the achievement.