Imperius sees Mephisto as an Abomination as decides to slay it/banish it for good at least for a time. Who wins this battle? Valor of Evil?
Imperius is The Archangel of Valor and the commander of the Heavenly Lost (Heaven army) and said to be the most powerful warrior in the entire creation after Anu, the Supreme being. Imperius never lost a battle against the Hell Hordes and he as well never felled to any demon. Imperius is the only know being in the creation know to fight The Lord of Terror multiple times and survived, different from all the others who fears him. By being Fearless, Diablo can't use the majority of his powers against him.
Imperius is extremely powerful. Able to defeat any enemies only with the force of his will or a single attack. He's also able to to travel FTL and it's said to destroy The Earth (sanctuary) like his brother Tyrael a thousand times using just a portion of his powers. His weapon, The Spear of Valor/Solarion which is said to be forged in The Hearth of a Dying Star like of his brothers can destroy any kind of corruption.
Mephisto is a perennial villain in the Marvel Universe, and is responsible for a number of evil acts, including capturing and holding the soul of Cynthia von Doom – the mother of Doctor Doom — until Doctor Strange and Doom free her to ascend to heaven. He was jealous of the worship of the fire demon Zarathos, so, posing as Satan, Mephisto creates the Ghost Rider by bonding Zarathos to Johnny Blaze. In one story where he battled Thor, he was shown to have various historical villains in his realm, such as Adolf Hitler, Blackbeard, and Attila the Hun.
Mephisto is an extremely powerful immortal demonic entity possessing magical powers and abilities gained by manipulation of the forces of magic. Mephisto is capable of using his power for a variety of uses, including superhuman strength, shape and size shifting, projecting illusions, manipulating memories, altering time,and is highly resistant to injury.