Undead is the dead body of the fallen (mostly soldiers or any being) that have been revived natuarally or through magic or other skills.
Rou and Mi are capable of summoning Undead en masse. When they are summoned, if enough mana is poured in, they can gain the color of the summoners's blessing. For example, Rou's are black while Kanami's are purple.
Purple Punk Ghoul