Sub Bosses are enemies with a purple nameplate, which display a Boss-like health bar in the corner of the screen when hit. Unlike real Bosses these monsters only have a single layer on their health bar. They only appear on Abyss difficulty.
Some Sub Bosses are enhanced versions of regular enemies. These have significantly improved stats and stronger attacks but their behavior is not noticeably altered from that of normal enemies, however killing them may still be required to open the next area. Other Sub Bosses are more fragile versions of a previous Boss. These are considerably more dangerous as they have the exact same attacks as their stronger incarnations and will usually be accompanied by a larger group of allies than in their first appearance.
All Sub Bosses drop the same crafting materials as their regular versions, with a higher drop rate.