Games Workshop has posted a PDF with several Armageddon scenarios for Warhammer 40K.
From their website:
The Third War for Armageddon saw every type of warfare as the greenskin menace wrestled with the might of the Imperium. Deadly cityfighting ravaged the great hives of Armageddon while the ash wastes saw vast tank battles as the might of the Imperial Guard’s armoured companies faced the frenetic menace of the Kults of Speed. The whole planet was embroiled in a relentless war of attrition with both forces stretched to the limit, each hoping the other would lose the will to fight first.