In Irish mythology, Aillen or Áillen was a monster from Mag Mell, the underworld. Every year at Samhain he would burn Tara with his fiery breath after lulling its inhabitants to sleep with his music. Fionn mac Cumhail, protected by a magic spear, defeated him and won the leadership of the Fianna.
The fertility goddess Aine is also sometimes known by this name.
Then Aillen would blow flames of crimson fire from his mouth, bright dancing flames that burned everything, from the four great ramparts, to the last blade of grass.
Aillen shot a flame of crimson fire from his mouth, but Fionn held up his four folded cloak against it.
He followed close on his heels, and as Aillen was going in through the doorway of the Sid palace, Fionn made a cast with his spear.
Aillen reached the stockade and shot a greenish flame from his mouth to the timbers.
Aillen wailed and fled for the fairy hill, its doorway open and emitting a green light.
The twenty years that Aillen had visited Tara corresponds approximately to the nineteen year cycle of the heavens with the intercalation of seven months, and the thirty golden rivets in Finn's spear to the days of the solar month.