What act of a mythical hunter-warrior of Irish mythology won him the leadership of the Fianna?
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.
Connla Cáem Connla Cáem ("the beautiful"), also known as Connla Cruaidchelgach ("bloody blade"), son of Irereo, was, according to medieval Irish legend and historical tradition, a High King of Ireland. He came to power after he killed his predecessor, and his father's killer, Fer Corb, and ruled for four (or twenty) years, until he died in Tara, and was succeeded by his son Ailill Caisfiaclach. The "Lebor Gabála Érenn" synchronises his reign with that of Ptolemy IV Philopator of Egypt (221–205 BC). The chronology of Geoffrey Keating's "Foras Feasa ar Éirinn" dates his reign to 319–315 BC, the "Annals of the Four Masters" to 463–443 BC.