Uaithne

The magic harp of Dagda, which could fly to him, if Dagda sang to the harp. The Fomorians had stolen Uaithne, but the harp flew back into his hand, killing nine Fomorians warriors along its flight path.
Uaithne, also known as "the Four Angled Music", was a richly ornamented magic harp made of oak which, when the Dagda played it, put the seasons in their correct order; other accounts tell of it being used to command the order of battle.
It would be really cool if when he played the Harp in battle his soldiers immediately knew what he wanted them to do, be it retreat, move to another location, etc. This would make his armies extremely co-ordinated and difficult to fight against. 
It also seems, that if the harp calls the Seasons, then it can be used to resummon Balor, but also to prevent Balor from leaving the battlefield.
Category:Mythical Items
Category:Celtic Objects
Category:Legendary Items