Mephisto by Blorgorize

Tier 1
[Q] Tool of Hatred
Each Regeneration Globe Mephisto receives permanently grants him 1 Tool of Hatred stack, granting his Host bonus effects while he is within them, based on their role:
(Minion / Mercenary): Max Health and Attack damage increased by +1 (+2 w/ Mercenaries).
(Attack-based Assassin): Max Attack damage against Heroes increased by +1.
(Ability-based Assassin): Max Ability damage against Heroes increased by +1.
(Warrior): Max Health increased by +4.5. When the Warrior Host loses Morlu

Classic Mephisto
Mephisto is the Lord of Hatred and revels in watching those around him scheme and war with one another. An enjoyment that would become detrimental during the Fifth Battle of the Diamonds, where the Burning Hells was on the cusp of victory against the High Heavens before the Great Evils bickered among one another.

As the Eternal Conflict ground to a halt with the disappearance of the Worldstone, Mephisto plotted to use the fledging Nephalem race and Sanctuary to bring Hell its final victory, instigating the Dark Exile so that he and his brothers could corrupt humanity from within.

Master Mephisto
The Lord of Hatred corrupted the Zakarum through his soulstone until he was later freed by his brothers and consequently slain by a specific Nephalem, who then went on to slay Mephisto