Aillen

Aillen or Áillen is a being in Irish mythology. Called "the burner", he is a member of the Tuatha Dé Danann who resides in Mag Mell, the underworld. According to The Boyhood Deeds of Fionn, he would burn Tara to the ground every year at Samhain with his fiery breath after lulling all the inhabitants to sleep with his music. This only ended with the arrival of Fionn mac Cumhaill, who inhaled the poison from his spear to keep himself awake and slew Aillen. The act won him the leadership of the Fianna.
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[Celtic] A malevolent beast from the Otherworld in Irish mythology.