If Tyreal is evil its not because he's Tyreal I could see him being Belial the lord of lies and Diablo II when you kill the Prime Evils it was just a fraud or hoax. The goal was to get the worldstone destroyed, possibly to remove the protection or w/e it did to allow the real prime evils to enter the world freeing them from their soulstones. basicly making what you did in DII a masquerade.
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