Theka the Martyr patrols around a room full of Scarabs. The Scarabs do not aggro unless you attack them.
This is a pretty normal battle. However, once Theka is near ~20% health he uses the ability Theka Transform, with which he becomes immune to both Shadow and Physical damage. Soon after this he makes the nearest Scarabs aggro, usually on the lowest group member (Usually around 5 Scarabs, so be careful if you're cloth). He stays immune for close to 30 seconds so just kill the scarabs then pound him.