S1E12: Battle of Assassin

Lugh and Dia's father come up with a plan to fake Dia's death and safely sneak her into to the Tuatha Dé region. Unfortunately, the arrival of Setanta MacNess thwarts the plan before they can enact it. As Setanta is the wielder of Gae Bolg, Lugh knows that he will not survive a direct confrontation with him, so he fires a magnetically propelled tungsten projectile into the atmosphere and stalls Setanta long enough for it to obliterate him. Afterwards, the Tuatha Dé family adopts Dia under the guise of being Lugh's younger sister, completing the mission. Sometime later, Lugh receives word that the Hero, Lord Epona Rhiannon, has arrived in the Alvan Kingdom.