Evan lived in Camelot during the reign of Uther Pendragon. When the curse of the unicorn fell on Camelot and it's people he sneaked into the castle's grain stores. When he was caught, he begged Arthur to give him food, saying he had children to feed. Arthur gave him a few grains and Evan told Arthur that he was merciful and generous, saying that it will 'bring it's own reward'. The next day water came back to Camelot. He was later seen in the Forest of Balor, when it was revealed that he was actually a thief. He started insulting Arthur about being weak and telling him he will be a bad king. Arthur rises to the insults, and kills him after a fight. Anhora appears and tells him he has failed the second test, as he could have easily ignored Evan, but instead killed him to protect his pride (The Labyrinth of Gedref).
It's likely that Evan is an illusion conjured by Anhora to test Arthur, because his body vanished when Arthur killed him, or possibly Anhora used magic to disguise himself, as after Arthur let him have the grain, he talked similar to Anhora, and when he disappeared, it was in the same way that Anhora used to teleport. However since it was never confirmed that Evan was actually Anhora in disguise, Evan most likely is a different person or an illusion conjured by Anhora.