Head of Annwfn
In The Spoils of Annwn, Arthur travels to his realm and steals his magic cauldron. Six hundred of Arthur’s warriors were slain in the expedition.
In non-Arthurian Welsh tales, the Head of Annwn is variously named as Arawn, Hagan, and Pwyll.
See also
Annwfn | The Legend of King Arthur
Source
Preiddeu Annwfn | Attributed to Taliesin, c. 900