The Undead (不死族, lit. undying race) are beings created from corpses given the black blood of the No-Life King.
Skeletons, zombies, and ghosts are not technically undead even if humans view them as such. The undead are creatures that have died but are not dead nor do they decompose. They have regenerative abilities that prevent them from being destroyed. Not even destroying the brain or burning them will stop them. It is thought that the No-Life King gave his black blood to dead bodies to turn them into the undead. With the No-Life King gone, the priests of the undead perform the "ritual of undeath" to undeathen corpses.
Those who become undead lose their previous memories and often pledge allegiance to the No-life king, however not all of them do and some even choose to work alongside humans (like Jimmy) or unaffiliated groups (like Arnold of Forgan).
Finally, the Five Princes are a group of special Undead who once were the No-Life King closest Vassal.