The Undead race (aka Reaper) is a class where formidable bosses can be found. Enemies that belong to this class are weak to Undead Killers. Notable Undead Bosses include Creepy Bogeyman, Dabih, and the Esper Anima. Undead Killer skills are a passive effect when equipped by a unit.
To find the killer gear for the Undead race click here to open the Killer Gear Finder
These trial bosses can be found in the Vortex. The Esper battles can found in the world map. See wiki page for more details.