In Le Morte d'Arthur, published in 1485 and popular for over 500 yearsever since,
Then the king gat his spear in both his hands, and ran
toward Sir Mordred, crying: Traitor, now is thy death-day
come. And when Sir Mordred heard Sir Arthur, he
ran until him with his sword drawn in his hand. And
there King Arthur smote Sir Mordred under the shield,
with a foin of his spear, throughout the body, more than
a fathom. And when Sir Mordred felt that he had his
death wound he thrust himself with the might that he
had up to the bur of King Arthur's spear. And right
so he smote his father Arthur, with his sword holden in
both his hands, on the side of the head, that the sword
pierced the helmet and the brain-pan, and therewithal Sir
Mordred fell stark dead to the earth; and the noble
Arthur fell in a swoon to the earth, and there he swooned
ofttimes. (...)
or if you prefer the Wikipedia summary,
Arthur charges Mordred and impales him with a spear. But with the last of his strength, Mordred impales himself even further, so as to come within striking distance of King Arthur, then gives a mortal blow to Arthur’s head.
This was reversed in the movie Excalibur (1981), perhaps because the filmmakers wanted to show Arthur fighting with the titular sword. Whatever the reason, the film shows Mordred striking the first blow and Arthur pushing himself along his son's spear to strike the second blow.
Mallory did not invent the Arthurian myths, and his major source for this material may have been “La Mort le Roi Artu,” also called “Lancelot-Grail”, a French rendition from about 1230. This pushes back the trope about 800 years.
However, Mallory is the source that established Arthur in English and American culture, and is the most direct inspiration for modern renditions of the trope.
