Mororis thought the Orks were deliberately humiliating the Astartes by imitating them and shouting, "For the Emperor."
He intended to humiliate them a hundredfold in exactly the same way.
Although his WAAAAAGH was loud, the Orks could not understand a word the Chief Librarian was saying.
Now, let us look at this from the Orks' perspective.
Our boss Worstan made a right flashy entrance, yelling at the red tin can who had just won, "Stinking humie, you're fighting me!"
Just then.
A voice came from the sky: "Slime Jumper is the Cosmic Painboy Wrench."
While the Orks stared in bewilderment, an enormous robot descended from above, only a tiny bit shorter than the Emperor Icon Mech.
This very waaaaagh-looking brute carried two enormous rectangular mesh boxes on its shoulders, which began blasting out incredibly loud nonsense.
"THE MUSHROOM SOUP AND CHOMPY BATTLESHIP IS MY BOSS!!!"
"KICK-LEG SIDE PONYTAIL TASTES BEST!"
"SNEAKY GO-HAEL STORM BOOMGUN!"
The Orks exchanged looks.
Our strongest and smartest Lord Worstan seemed to understand something. He decided this thing was mocking them, so he imitated the enemy's words to mock them back.
And so the Astartes heard things like "Die, Horus!" and "Traitor scum!" They were equally speechless.
With remarkable mutual understanding, both sides began competing over who could shout louder.
Everyone else was left inexplicably even more eager to fight.
There was no helping it. When the Astartes heard the other side repeatedly shouting the traitor's name, all they could think of was the Horus Heresy.
As for the Orks... they always wanted a fight.
Kazhaniv: Type "1" to check the Chief Librarian's mental state.
Annos: You have to understand, after spending over a hundred years inside a Dreadnought, staying sane would be the abnormal thing.
Kazhaniv: So our Chapter has an Ork now? Are we