|Mordred spears Arthur in the movie Excalibur|
Gueith camlann in qua Arthur et Medraut corruerunt.
The Battle of Camlann, in which Arthur and Medraut (Mordred) fell.
It has been noted that this short text does not make it clear that the two fell in combat against each other or alongside each other as allies against a common enemy.
Geoffrey of Monmouth, in his Historia Regum Britanniae (History of the Kings of Britain) provides the first account unambiguously describing the two as enemies. He describes Mordred as a traitor who seizes the throne while Arthur is on the continent, prompting Arthur to return and fight Mordred in the Battle at Camlann. Arthur kills Mordred but is severely wounded in the exchange and goes off to Avalon in hopes of healing.
I will argue that the available textual parallels make it clear that the reference in the Annales Cambriae implies that the two did not fight as allies.
Textual Parallels to the Annales Cambriae
The verb corruō, corruere, corruī means "to fall together, to fall," and figuratively, "to die."
It can describe an object falling, as in this reference by Suetonius:
statuae equestres, cum plurifariam ei ponerentur, fractis repente cruribus pariter corruerunt.
The Equestrian Statues, which were erected for him in several places, fell at the same time, their legs suddenly having been broken.
(Suetonius, Vitellius 9)
The adverb pariter in this quote can be translated as 'at the same time', 'together', equally, or 'in like manner'.
St. Jerome uses the verb corruere in his Vulgate translation:
omnesque ante eum in terra pariter corruerunt
And all fell together before him on the ground.
What is curious about Jerome's translation here is that there is no word in the Hebrew which corresponds to pariter:
וַיִּפְּלוּ לְפָנָיו אָרְצָה. (vayyippalu lifanav 'artsa)
And they fell before him to the ground.
Given the use of pariter alongside corruerunt in the quote from Suetonius above, as well as Jerome adding it here, it would seem the two words work together idiomatically.
The verb corruere can also appear without the adverb pariter, as in this Letter from Cicero:
tabernae mihi duae corruerunt reliquaeque rimas agunt
Two of my shops have fallen down and the rest are cracking.
(Cicero, Letters to Atticus 14.9)
Presumably one of Cicero's shops fell from disrepair, then, some time after, another one fell. If they had both fallen simultaneously, perhaps as a result of an earthquake, then Cicero would have employed the adverb pariter to express that pairing.
The closest parallel I can find to the quote in the Annales Cambriae is from the Venerable Bede's History of the English Church:
Nam egressi contra gentem Geuissorum in proelium, omnes pariter cum sua militia corruerunt.
For having gone out against the nation of the Gewissie in battle, all fell together with their military.
Bede, Historiam Ecclesiasticam Gentis Anglorum 2.5)
Again, we see here the pairing of pariter and corruere.
The Deafening Absence of Pariter in the Annales Cambriae
The quotes in which the verb corruere is paired with pariter emphasize either the sudden mutual falling of objects or the mutual falling of people in solidarity. St. Jerome adds the adverb to describe the scene of Joseph's brothers in concert falling to the ground. Bede's quote employs pariter to express the fact that the leaders fell together, alongside their military.
Given the abundant examples of corruere occurring with pariter, I conclude that the absence of it denotes people or things falling either unevenly or not in common cause. While the statement "Arthur et Medraut corruerunt (Arthur and Medraut fell) is technically ambiguous, it would have been understood as implying that Arthur and Medraut died, but not together or as comrades.
Scholars debate how much of the Arthurian Legends were Geoffrey of Monmouth's literary creation. My conclusions in this post indicate that, at least as regards Mordred, what Geoffrey of Monmouth presents in his Historia Regum Britanniae is likely not his invention. He is fleshing out stories that have considerable detail in either written or oral form before he published his work. He may have brought these legends to life, but he did not compose them.