Mephisto

Mephisto was the first Prime Evil to be captured, and his stone was buried in a vault beneath the Temple of Light in Kurast. A third of the Horadrim stayed to guard this tomb until the remaining two Brothers were captured. Though the cost was high, all of The Three were eventually captured.

Mephisto finally managed to overcome the powers of the Soulstone that imprisoned him enough to influence and corrupt the local priesthood, known as the Zakarum. The Demon then had the priests work spells to shatter the Soulstone into seven shards, breaking the mystic seal and freeing the Lord of Hatred. Mephisto drove the six smaller shards into the left palms of six Zakarum archbishops who were then charged with bringing him nourishment in the form of human sacrifices while running their temple. The largest stone pierced the hand of the Que-Hegan, the highest divine authority of the Zakarum faith. It was this body that the Demon used as his manifestation on the mortal plane, warping the mortal’s form into an approximation of his true visage. For years, the corrupted religious leaders of the Zakarum have kept Mephisto supplied with information and misguided followers to carry out his plans of conquest. He eventually met up with Diablo, and the two began to work to enslaving the human race. However, Mephisto was later killed by the same adventurers (or adventurer) that would go on to kill Diablo. Mephisto's Soulstone was destroyed in the Hellforge after his defeat, ensuring his banishment from Sanctuary.

Mephisto is considered the most intensely evil brother of the three Prime Evils (and is also the oldest). So overwhelming is his evil that his malevolent influence can bring the dead back to life; these zombified beasts are filled with such anger for the living, that not even death can quench their desire for pain and suffering. Mephisto may also be viewed as the most intelligent and calculating. Over the years, after having ensorcelled the Zakarum, he used his newfound power to bring the lands of Sanctuary under his heel through the martial power of warped Paladins, and sent the Zakarum deep into Westmarch, where his unwitting servant, King Leoric, would take over Tristram.

Auriel

It is hinted that Mephisto and the Archangel Auriel may have once been lovers.

"'We like you, made false assumptions about this place, false assumptions about the creatures spawned by both our kind.' A blazing red eye materialized, then winked in the direction of Auriel, who utterly ignored it." (Mephisto, pg 332 of The Veiled Prophet)

Also, when the Kiss of Mephisto (also known as the 'Blood Marble' that resurrects the Morlu) is first mentioned, it is said that it was named 'Kiss of Mephisto' because it was originally a gift for someone else, other than Lucion. If Lucion doesn't approve of the 'kiss', then it doesn't seem to be the sort of kiss that a father gives to his son. Therefore, it also seems unlikely the type of kiss that is from a father to his daughter, so it is also unlikely that it is aimed at Lilith. If not Lilith or Lucion, then Mephisto must have been involved with someone else. Tying in with his wink at Auriel, it appears that Auriel was who the 'kiss' was targeted at.

"Turning his eyes back to the cataclysmic sight below, Lord Lucion gestured toward the very center, where the worst of the carnage had and was still taking place. In the midst of everything, a black gemstone nearly as large as a man sat embedded in a triangular column of red-streaked marble. “Blood marble,” it was named, naturally. The stone was called by Malic’s master the Kiss of Mephisto, although the cleric had, from past comments, reason to believe that it had once been named for another of whom Lord Lucion would not speak." (Birthright)

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