The demon Atakhan was killed by the darkin Zaheen, who has taken Xin Zhao as his host, either through the sheer strength of his powers as a darkin or that his magic is specifically to celestial/sun magic, or more likely a combination of both.

The point is LeBlanc's pretty chuffed at this revelation because it means there's a way to destroy Raum, the demon of secrets, and more importantly a way to kill Swain who had Raum bound to him.

The final cinematic then cuts to this last image of a three-eyed raven and a boat, which is likely in reference to one of the last scenes of Arcane, where Mel's ships set off for Noxus, followed by Swain's raven.

This shot is both the answer and the dilemma for LeBlanc. Raum, and thus Swain have a weakness. The only person in the cast with remotely similar sun powers in the cast like Zaheen's sun/celestial magic is Mel.

Mel has been repeatedly flanked by Swain's raven from Act 3 of s2, up until the moment she passes the Noxtorra and his ravens caw for her in the background. So if it weren't enough that Swain is very clearly targeting Mel for recruitment to his side, Mel also wants LeBlanc dead for very obvious reasons.

There's also the underlying irony that this discovery is a great reference of Mel for Swain. Swain knows Raum will eventually take control of him, he WANTS his inner circle to be people he believes would be capable of killing him when Raum inevitably subsumes his will. And now LeBlanc's confirmed yes this new mage in Noxus can explicitly fulfill his literal and very specific death wish when he needs it to happen, which would theoretically prevent his worst case scenario from mucking up his agenda.

I'm never touching demacia again out of sheer disgust for that augment. I saw a grand total of 5 times since the very beginning and one time was on pic poc trial.