Erc mac Cairpri is a character from the Ulster Cycle

Ulster Cycle

The Ulster Cycle , formerly known as the Red Branch Cycle, one of the four great cycles of Irish mythology, is a body of medieval Irish heroic legends and sagas of the traditional heroes of the Ulaid in what is now eastern Ulster and northern Leinster, particularly counties Armagh, Down and...

 of Irish mythology

Irish mythology

The mythology of pre-Christian Ireland did not entirely survive the conversion to Christianity, but much of it was preserved, shorn of its religious meanings, in medieval Irish literature, which represents the most extensive and best preserved of all the branch and the Historical Cycle. There are...

. His is the son of Cairpre Nia Fer, king of Tara

Hill of Tara

The Hill of Tara , located near the River Boyne, is an archaeological complex that runs between Navan and Dunshaughlin in County Meath, Leinster, Ireland...

, and his wife Fedelm Noíchrothach

Fedelm Noíchrothach

Fedelm Noíchrothach , also known as Fedelm Noíchride , is a daughter of Conchobar mac Nessa in the Ulster Cycle of Irish mythology....

, daughter of Conchobar mac Nessa

Conchobar mac Nessa

Conchobar mac Nessa was the king of Ulster in the Ulster Cycle of Irish mythology. He ruled from Emain Macha .-Birth:...

, king of Ulster

Ulaid

The Ulaid or Ulaidh were a people of early Ireland who gave their name to the modern province of Ulster...

. After Cairpre is killed by the Ulster hero Cúchulainn

Cúchulainn

Cú Chulainn or Cúchulainn , and sometimes known in English as Cuhullin , is an Irish mythological hero who appears in the stories of the Ulster Cycle, as well as in Scottish and Manx folklore...

 in the battle of Ros na Ríg, Erc is installed as the new king of Tara, is given Cúchulainn's daughter Fínscoth in marriage, and swears allegiance to his grandfather Conchobar. However he later conspires with Lugaid mac Con Roí

Lugaid mac Con Roí

In the Ulster Cycle of Irish mythology, Lugaid mac Con Roí was the son of Cú Roí mac Dáire. He was also known as Lugaid mac Trí Con ....

 and others to kill Cúchulainn. He is then killed by Conall Cernach

Conall Cernach

Conall Cernach is a hero of the Ulaidh in the Ulster Cycle of Irish mythology. He is said to have always slept with the head of a Connachtman under his knee. His epithet is normally translated as "victorious" or "triumphant", although it is an obscure word, and some texts struggle to explain it...

, Cúchulainn's foster-brother, in vengeance. When Conall takes his head back to Tara, his sister Achall

Achall

Achall, daughter of Cairbre Nia Fer, king of Tara, and his wife Fedelm Noíchrothach, is a minor character from the Ulster Cycle of Irish mythology...

dies of grief.