In a famous episode from the Táin Bó Cúailnge, or Cattle-raid of Cooley, demigod Cú Chulainn - the most terrifying creature in Irish mythology - and his foster-brother Ferdiad find themselves on opposite sides.
Medb, queen of Connacht, launches the raid in order to steal a renowned stud bull, but Cú Chulainn has invoked the right of single combat at fords and is single-handedly defending Ulster from her army. Mebd knows Ferdiad is the only warrior capable of defeating Cú Chulainn, but Ferdiad doesn't want to fight his best friend.
Then did Medb despatch the druids and the poets of the camp, the
lampoonists and hard-attackers, for Ferdiad, to the end that they
might make three satires to stay him and three scoffing speeches
against him, that they might raise three blisters on his face, Blame,
Blemish and Disgrace, if he came not with them.
They set about goading him and plying him with alcohol to persuade him to fight, even offering him the Queen of western Erin. And eventually, reluctantly, he goes.
After three days of gruelling combat in a ford, using every weapon imaginable, Cú Chulainn is almost beaten. He signals to his charioteer, who floats the Gáe Bulg down the stream to him. Throwing a spear at Ferdiad's chest to distract him, Cú Chulainn catches the Gáe Bulg in the fork of his foot and lets fly from underneath into his best friend. Its barbs open, spreading throughout his body and killing him.
While the charioteer cuts and hacks the weapon free from the dead man's body (I can almost hear him whistling as he works!), Cú Chulainn laments:
Ah, Ferdiad, betrayed to death.
Our last meeting, oh, how sad!
Thou to die I to remain.
Ever sad our long farewell!
All was play, all was sport,
Till came Ferdiad to the ford!
Dear the shaft of gold
I smote on the ford.
Bull-chief of the
Braver he than all!
The whole episode is here.
[Cú Chulainn is similarly grief-stricken after killing his son Connla, but that is a story of the Sohrab and Rostam or Arjuna and Babruvahana type, in which the identity of the son is only revealed as he dies.]