Aatrox Whispered, "One Day Within Us..." [Theory]
When he ults he says either
- Witness Truth!
- Behold!
Which can, faintly, be heard globally. But....during that ult you can also hear sinister whispers. It sounds like speech, but seems impossible to understand. Until, as you see in the video, they are played in reverse. Kind of an easter egg probably alluding to his daemonic appearance, and the idea of metal records having secret satanic messages while played backwards,
https://www.youtube.com/watch?v=LCaXnG7y368
Now, what is interesting is that there are not ONE, but at least TWO different voices-possibly more.
http://tvtropes.org/pmwiki/pmwiki.php/Main/VoiceOfTheLegion
One is Aatrox current voice actor, with lines taken directly from his normal move/attack quotes (albeit some of them are stressed a little differently). There is another male voice that says
- Die before me.
- One day!
These are two quotes that Aatrox himself doesnt say at all. The latter of the two overlaps with "War reveals what is within us" to sound like
"(War reveals-) One day (within us)".
I have some theories on this, that
- These voices are Aatrox himself, and also the "voice" of his living Sword.
- These voices may include the voices of the spirits of those he has killed in battle, and whose blood his sword has drunk, possible fused together into some hive-mind composite spirit.
- These whispers carry subliminal messages that Aatrox uses in combination with his fighting style to mind control people.
His background says
He wields his massive blade with grace and poise, slicing through legions in a style that is hypnotic to behold.
and
Soon, the warriors found themselves imbued with a potent thirst for battle. They followed Aatrox into the fray, each fighting with the furious strength of ten until they had won a most unlikely victory.
Aatrox vanished after that battle,but the Protectorate army's newfound fury did not.
"I know what lurks in the hearts of men."
Their surprising triumph led to many more until they were able to finally return home victorious. Their countrymen hailed them as heroes, but though they had saved their entire civilization from destruction, darkness lingered in the mind of each warrior. Something within them had changed.
Over time, their memories of battle faded, only to be replaced with a grim revelation: their acts of heroism were, in fact, brutal atrocities committed by their own hands.
http://tvtropes.org/pmwiki/pmwiki.php/Main/GoMadFromTheRevelation
That was the moment that Aatrox's influence wore off, and men came to grips with their own savagery and the dark ecstasy of endless carnage. I'm willing to bet, that in time, their enemies would seek revenge.
"Sow the seeds of strife." 
However, while some civilizations chose to mythologize these battles, and do things up in cheery noble colors. Others made appropriately blood stained Ionian wood block prints, sacrificial knives in the Freljord, and more...which eventually became reviled but never destroyed, only misplaced or buried.
http://riot-web-static.s3.amazonaws.com/images/news/April_2013/post/ez-desk.jpg