Aillen Trechenn

Aillen Trechenn is a legendary creature from Cruachain, Roscommon, Ireland, dated to ~500 BC (Classical antiquity).

Aillen Trechenn was the name of a monster in the folklore of Ireland. In Irish Gaelic, this name means "Triple-Headed", for it was said that this creature had three heads extending from its gigantic body. According to one source, it either inhabited a cave in the mountainside, or an ancient burial mound at Cruachain in Roscommon County.

This creature detested humans, especially followers of Emain Macha and the inhabitants of the ancient stronghold of Tara. On one of its tantrums, on the eve of the old Celtic festival of Samain (now celebrated as Halloween), that Aillen Trechenn attacked its local community, bent on destroying the people of Ireland. It was finally brought to its doom by Amaigin( Amergin Glúingel), a warrior bard.