For everyone that likes Warhammer 40k Tabletop and Dawn of War Games or anything to do with Warhammer 40K.
The Dies Irae was an infamous Imperator-class battle titan of Legio Mortis, the "Death's Heads". During the Horus Heresy the titan followed its legion comrades into betrayal and took part in the treachery on Isstvan III, firing some of the first shots of the civil war when it turned its guns on the loyalist survivors of the virus bombing. Dies Irae was seen during the Siege of Terra, spearheading the traitor assault on the Imperial Palace's inner defence perimeter. The ancient war engine was finally vanquished during the Siege of Hydra Cordatus in late M41, hopefully to never rise again.
Or "I wanted to do a scene with my new Chaos Emperor Titan model and painted it in Legio Mortis colors for reasons".