system
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human
[Diablo franchise] Why do Malthael’s angels look so different from other angels?

In Diablo III we learned that Malthael disappeared for a while, a host of angels went to look for him and also disappeared, then centuries later reappeared to cleanse Sanctuary of demonic taint, but the angels all looked like demons themselves – what gives?

Was there some form of corruption?  Malthael doesn’t have the normal wings of an angel until the moment you kill him, then the skeleton wings briefly disappear and are replaced with angel tentacle-wings as if you’d just released him from something corrupting…
gpt
After Malthael abandoned his post among the Angiris council, he wandered through Pandemonium fortress seeking answers to his burning questions about good, evil, mortality, and the end of the eternal conflict. He knew humanity would be the key to the end, all the angels and demons know this, that's why they fight over Sanctuary so much. But Malthael felt that there was something else to it all. 

After the Nephalem destroyed Diablo and stood among the High Heavens, which no mortal had ever done, he realized humanity was an abomination of both sides. The end of the eternal conflict meant the end of angel-kind. (and also demons, but he's fine with that) So he plunged into the deepest depths of Pandemonium and found the site where the worldstone used to rest before it was stolen and placed inside Mount Arreat. In that place, a vortex of souls swirled around, and Malthael used their power to increase his abilities. He crowned himself aspect of Death, and took on his new appearance. His followers were granted portions of the same power, and so they took on a similar glow. 

Most of his minions look much the same as they did when they were angelic, but the soul-energy they use defines their bodies, so they don't look like empty suits of armor anymore. Other lesser minions, like the raised up, yet empty, husks of human bodies, are very similar to some of the dark magics that the demons use. (it's probably the exact same spell, just used hypocritically)