According to tradition there are 13 treasures of the Island Of Britain. These are known from old texts from the 15th and 16th centuries.
Many of these are not known to be connected to King Arthur, but a few are directly related to him or to his cousin Culhwch in the stories of old.
Cauldron of Dyrnwch
In particular the Cauldron of Dyrnwch the Giant which is said to reveal men who are cowards and men who brave. By the simply virtue of boiling rapidly if cooking meat the belonged to a brave man – but it would not boil for a cowards meat.
In the tale ‘Culhwch ac Olwen’, Diwrnach’s cauldron (a symbol that crops throughout Celtic Mythology) is among the items Culhwch must gather for a banquet. King Arthur himself makes a request for the cauldron to Kind Odgar. But a request for such a prize is refused.
So Arthur travels to Ireland to seek out Diwrnach to make a personal plea for the treasure. But again he is refused.
At which point Arthur’s champion takes the cauldron on Arthur’s behalf and Llenlleawg slays Diwrnach and all who defend him. More fighting ensues but Arthur and his party hold off the Irish, take to their ship and leave with the cauldron safely aboard.
The Mantle of Arthur
A true cloak of invisibility, King Arthur’s Mantle allows those beneath it to see out yet those outside to see nothing but thin air. This treasure is known from multiple sources, including in Culhwch where it curiously remained undescribed in detail.