Caladbolg sometimes written Caladcholg ("hard blade"), is the sword

Sword

A sword is a bladed weapon used primarily for cutting or thrusting. The precise definition of the term varies with the historical epoch or the geographical region under consideration...

 of Fergus mac Róich

Fergus mac Róich

Fergus mac Róich is a character of the Ulster Cycle of Irish mythology...

 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...

. Spelled Caladcholg, it is also associated with the more obscure Ulster hero Fergus mac Leda, suggesting a conflation of two legends. It was said to be a two-handed sword that made a circle like an arc of rainbow when swung, and to have the power to slice the tops off hills and slaughter an entire host.
During the Táin Bó Cuailnge

Táin Bó Cúailnge

is a legendary tale from early Irish literature, often considered an epic, although it is written primarily in prose rather than verse. It tells of a war against Ulster by the Connacht queen Medb and her husband Ailill, who intend to steal the stud bull Donn Cuailnge, opposed only by the teenage...

, Ailill mac Máta

Ailill mac Máta

Ailill mac Máta is the king of the Connachta and the husband of queen Medb in the Ulster Cycle of Irish mythology. He rules from Cruachan .-Family background, marriage and offspring:...

 takes Caladbolg away from Fergus mac Róich when he discovers Fergus' affair with his wife Medb

Medb

Medb – Middle Irish: Meḋḃ, Meaḋḃ; early modern Irish: Meadhbh ; reformed modern Irish Méabh, Medbh; sometimes Anglicised Maeve, Maev or Maive – is queen of Connacht in the Ulster Cycle of Irish mythology...

. He gives it back when the Ulstermen rally against his armies. Fergus wreaks havoc against Ulster's forces with his blade, but 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...

 convinces him not to kill 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:...

. Fergus strikes the Three Great Strokes on three small hills instead, blasting off their tops.

Caladbolg is thought to be a source or analogue of King Arthur

King Arthur

King Arthur is a legendary British leader of the late 5th and early 6th centuries, who, according to Medieval histories and romances, led the defence of Britain against Saxon invaders in the early 6th century. The details of Arthur's story are mainly composed of folklore and literary invention, and...

's sword Excalibur

Excalibur

Excalibur is the legendary sword of King Arthur, sometimes attributed with magical powers or associated with the rightful sovereignty of Great Britain. Sometimes Excalibur and the Sword in the Stone are said to be the same weapon, but in most versions they are considered separate. The sword was...

, which in early Welsh

Welsh mythology

Welsh mythology, the remnants of the mythology of the pre-Christian Britons, has come down to us in much altered form in medieval Welsh manuscripts such as the Red Book of Hergest, the White Book of Rhydderch, the Book of Aneirin and the Book of Taliesin....

is called Caledfwlch.