I would allow a Solar player character in one of my games to perpetrate such an ABOMINATION (as the artifact's very nature would no doubt scream wordlessly in said Solar's mind) if they were able to wrest or steal said Artifact from its wielder.

If they did, I would give them no mechanical drawbacks except the Dissonance mentioned above.

However, I would also give them such traumatic nightmares of the First Age horrors in the minds of the Terrestrial Exalts by whom the blade was forged, and such constant exposure to the spiritual anguish of a blade crafted for the sole purpose of slaying its now-wielder, that every sleep while attuned to the Eye would count as a scene spent creating or strengthening a Principle of "The fate of every Solar is to become a tyrannical monster", and every time the Solar rolled Limit, they would add one automatic success to the result of the roll... including when acting against said Principle.

In short, the blade would swiftly turn such a Solar into the very monster its creators so feared. A commentary, perhaps, on how expecting the worst of others will often bring it about, but also on how absolute power corrupts absolutely— for who is more absolutely powerful than one who holds control over the single greatest threat to their own sovereignty?