Caladbolg

The magical rainbow sword Caladbolg was a relic from the bygone ages. Crafted by Brigid, Who was a priestess of the goddess Ritona, and given to Goibniu during the great cleansing. The sword is said to be able to create arches that look like rainbows and can cut through anything they pass, even mountains. This was used by the Goibniu who still believed in the old gods as he tried to defend against incoming attacks from the Tonash Clan armies. On the fourth week of the siege on Cil Mochloa He was betrayed by his own son, Mogon, who hide the sword while his father slept, And told the armies of the dwarfs to come and attack. For this Mogon will eventually be rewarded in godhood, as his city burns from his actions. One more hero held the sword after Goibniu, and it was Tera mac Leda, A descendant of Brigid who went into the dived into the lake near Cil Mochla to find the sword. She would find out that the Selkie king took ownership of the sword and that it could not be used underwater because a rainbow could not form there. Ever since it fall into the Selkie hands they used it as a ritual sword hang above the throne, as using swords underwater is also inefficient. Tera mac Leda plead from the king to get the relic she thought she deserves by birthright, but the king insisted to know why it's so important. Upon telling him of its power the king tricked Tera to follow him as he held the sword and plunged both of them into a portal to the Otherworld, Both were never seen again.