Arthur kills emperor

When the emperor saw his nephew mortally wounded,
he charged at Kay the Seneschal and struck him so hard
that his lance penetrated his body; he struck him down so
seriously wounded that Kay lived [209] only three more
days. Then the emperor drew his sword and advanced on
Girflet. He gave him such a blow on the helmet that Girflet
was completely dazed; unable to stay in the saddle, he fell
from his horse.
King Arthur saw these two blows and realized that the
assailant was the emperor. He spurred in that direction and
struck him on the peak of his helmet with his bright and
sharp sword, with such strength that the emperor felt the
cut of the sword down to his teeth. He withdrew his sword,
and the emperor fell to the ground dead; that was a pity,
because the emperor was a good knight and a young man.

The Vulgate Cycle