This was playing in my mind when the dreadnought got stuck in one of my friend's playthrough. I destroyed so many Thousand Sons while the dude playing Titus got beat up by Tzaangors.
This is when the power trip kicks in after ripping the first enemy of a horde in half. Then the jump pack ignites and ya hurl yourself right into the middle of the horde.
Head cannon is Rowboat Gorillaman accidentally found the Emperor's ancient mp3 player deep in the archives and got the song stuck in his head. He began singing this song to himself when he fights and the ultramarines, emulating their gene father, sing this "battle hymn" to give them strength in a fight.
Since the emperor is pretty much immortal and has lived like forever I am now imagining him during the 80's as this rock/punk dude I mean he even has a hair to go with it hahah
"Brother, what feelings do you feel when you slaughter the vile heretics and xenos? It better not be heretic anger.” “Throne forfend Brother. A sense of calm overcomes me and a tinge of pity."