Aatrox was once a wielder-weapon relationship long ago, similar to Kayn and Raast, but that wielder failed to overcome the Darkin within the blade and was consumed entirely, leaving only what we see as Aatrox behind.
EdIT: Okay, not new. More like, solidified.
That theory isn't new. It's been floating around for a while now that the Aatrox is actually the sword and not the one wielding it.
Same goes for Varus. It's been fan theoried for years that Varus owns a Darkin. Kayn and Rhaast pretty much confirm that this is the case.
might actually become a darkin boys! HYPE!!
plot twist; aatrox is actually the man and the sword was the host
Or they could have willingly submitted. We may never know.
Wait... if is theorized to have a Darkin weapon... and Kayne has a Darkin weapon... and they're both from Ionia...
Could the original wielder that 
was previously be from Ionia as well?
Are Darkin native to Ionia like Watchers are native to Freljord?
Riot better debunk or confirm this. This theory has been out for too long.
