Something to Note About Aatrox: He Can Drop His Weapon
Correct me if I am wrong, but it's been stated with Kayn and Varus that they're weapons are tied to them almost like an additional organ. If they drop their weapon for an extended time, they basically die from what I've read. Aatrox in the teaser just left his sword over there impaled in some guy while he was off doing something else, which means he has completely dominated his wielder or something similar in the state of his freedom. The new lore for Aatrox might be our first look at what a Darkin can do in their prime.
And yes, while I know his current lore makes him out to be pretty legendary, it's also old lore and never really describes the full extent of what he can do.