Okay, so, in that universe, Mar-Vell struck a deal with some kind of evil cancer-gods and it kept him from dying. They did this freaky ritual that destroyed the cosmic abstract of Death in their universe, meaning nothing could die, and they basically filled their whole universe up with creepy undying stuff, so they broke into our universe and tried to do the same thing, but thanks to Thanos, our Death got to Mar-Vell before he could get to her and she vaporized him.
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Okay, so, in that universe, Mar-Vell struck a deal with some kind of evil cancer-gods and it kept him from dying. They did this freaky ritual that destroyed the cosmic abstract of Death in their universe, meaning nothing could die, and they basically filled their whole universe up with creepy undying stuff, so they broke into our universe and tried to do the same thing, but thanks to Thanos, our Death got to Mar-Vell before he could get to her and she vaporized him.