It was an accident. Accidents, lamentable though they may be, happen. Polemgaos knew the risks of entering into battle, knew the limitations of his defenses. I am kind of sad that his immortality was a sham, though, he seemed rather cool; there was just so much history that he carried with him as closely as his armor, so many titles. The Emissary of the Burning Tree, which presumably gave him access to the Writ of the Flaming Bough; what kind of feats result in earning a perfect defense? The little things like that highlight that he was his own man, who had has his own loves and travails and great deeds. We ended that.
But you know what? Ulyssian is going to have to end a lot of people before this is done. Had he known the truth, Polemgaos would have slaughtered him without so much as an instant of remorse. Even this, we can turn towards our advantage; Ambition permitted it, after all, so Polemgaos' death can serve our interests. Let's not squander the fame to be gained here.
[X] "Don't worry, I struck you with the flat side of the- hm. Hmm."
Edit: Hmm, on second thought the write-in is a decent compromise option that benefits from an effectiveness bonus.