Eda Elyn Mawr
The British Museum Harleian MS 4181 (entry 42) gives this as the name of Arthur's killer at Camlann.
A Welsh Triad tells us that the warriors Peredur and Gwrgi - elsewhere given as Arthur’s warriors - were killed fighting Eda Great-Knee at Caer Greu. This defeat was attributed to the desertion of Peredur’s and Gwrgi’s warriors just prior to the battle.
Eda’s character is likely based on an actual eighth-century Northumbrian king.