Maseki Gourmet: Mamono no Chikara o Tabeta Ore wa Saikyou! - Vol. 7 Ch. 38 - Adventurer's Town, Baltic

[MangaDex]

 

[colorlesseatstomboys]

Something I read in the comments before makes it easy to guess.

Spoiler: About Dullahan and Elder Lich and the First King

It talked about Dullahan and The Lich's son and foster daughter. The daughter killed both the parents and the son then defeated the daughter (the demon lord). So it's safe to assume that the 1st King is Dullahan and The Lich's son, explains the mingling with other races too.