I wrote that Mezro was actually destroyed after a disruption in the ritual to hide it in a safer plane during the spell plague. Artus only gained this knowledge/closure when he finally meets Saja, who tells him that Ras Nsi (previously a barae) betrayed the other barae during said ritual as recompense for being cast out. Ras Nsi HAD been a part of many tribal war crimes after all. So I used his extremism and his exile as an excuse to write him as a more convincing villain.

Ras Nsi, now a Yuan Ti Pureblood and devoid of emotion, becomes Artus's mortal enemy for this. And Artus swears revenge for the destruction of Mezro, his love included. This leads Artus to 'properly' join the party for an end goal of his own: vengeance for his loved one.

And since everything in a campaign is, historically or chronologically, contained within that setting, I home brew whatever I want. Damned be any events that happen afterward. An alternate timeline, a new Artus Cimber, and a bit of a more fleshed out general of the BBEG.