Errors in the archives

Totally different weapon. They just happen to share an ability by coincidence.

Also, CMC got Mugendai-Yaiba and Kyuukyoku-Yaiba the wrong way around compared to what clues in the comic imply. The comic establishes the following:

1. Mugendai-Yaiba the Infinity Blade is the sword Eternion created used during the First Eternal War.
2. Kyuukyoku-Yaiba is the sword which the other Eternals created and used against Eternion during the First Eternal War, which Eternion took for himself for the Second Eternal War (the one we see in the comic). This sword is also described... somewhere... to be the Terminal Blade.
3. Argent's unnamed sword was forged to be similar to the one Eternion held during the "war", which pretty clearly refers to the First Eternal War (what I call the Second was really just a series of assassinations and a boss fight), and it deals damage according to the target's power. Thus, Mugendai-Yaiba logically also deals damage according to your enemy's power.
4. Kyuukyoku-Yaiba's ability is never mentioned in the comic, but it's not out of line to assume that it's similar to Ker's unnamed sword in that it deals damage according to your own power.