No-Life King

The Undying King (不死の王, Nō-Raifu Kingu; lit. Immortal King) is a mysterious and powerful figure who once ruled Grimgar over a century ago. After his supposed defeat, his curse of the Undead still lingers in Grimgar.

Souma's clan, the Day Breakers, have announced that there are indications in the lands of the Kingdom of Ishmael of the King's return.

The real name of the No-Life King appears to be Enad George, as stated by Jessie Smith, who apparently knows a lot about history of Grimgar.

Lore
While his lore is still unclear.

Undead
According to Honen, Undeads are the result of any corpse is not properly cremented within three days. Dark Spell are less effective on them, they are almost restaint to outer actions, and will continue to fight until they are hit by Dispel.