Flamerium
The thieves watched in horror as their fallen comrades stood up once again, but the look in their eyes wasn't the same. The decapitated skulls reattached to their bodies, a thin line forming on their necks where they were once cut.
"Roar!" the reanimated thieves ran towards their former friends with their knives and daggers held forward.
Those without weapons directly entered melee range, while the remainder stood back and casted the respective Magic they had awakened during their life. Since he didn't have to worry much about hurting himself or others, William ordered them to cast their largest-scale area of effect Spells.
The reanimated thieves didn't recover any Mana from the moment they died, but thankfully William had killed them fast enough, so their Reserves were still mostly full.