Which king owned the sword Caladbolg?

The king who owned the sword Caladbolg was Fergus mac Róich.

Caladbolg is a magical sword in the Ulster Cycle of Irish mythology. Its name is commonly interpreted as “hard cleft” or “hard blade,” and it is associated with Fergus mac Róich, a powerful exiled king and warrior. The sword was said to grow as large as a rainbow when swung, making it far more than an ordinary weapon.

In the Táin Bó Cúailnge, Ailill mac Máta takes the sword after catching Fergus with Medb. Fergus disguises his loss by carrying a wooden replacement. Ailill eventually returns the real weapon before the final battle, allowing Fergus to reclaim it.

Fergus is prevented from using Caladbolg against Conchobar mac Nessa, so he uses it to slice the tops from three hills instead. The sword is sometimes spelled Caladcholc, and its name is often discussed alongside similarly named weapons such as Excalibur and Cú Chulainn’s Gáe Bulg.