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But Diablo died in Diablo II.Maybe he was just revived by an ancient Wizard or so in Diablo III,i mean how can you tell?Blizzard didnt say anything.
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Think about it, between diablo 1&2 he was dead aswell. And at the end of the game, in a cinematic, the guy says that only one soul stone was left, and he gave it to someone he belived was tyrael, but it wasnt. And i think thats how diablo comes back.
According to what decard cain says, diablo was sealed in the soulstone before, and maybe he is again, and that way it is possible to bring him back.
Im not really good with the diablo lore but this is how i think it is.