Technically, she didn't die on Angel. She did die twice on her own show and the second time it was referenced on Angel. The first time, she was killed by the Master and given mouth-to-mouth by Xander and thus brought back to life. At the end of Buffy's fifth season (and the end of Angel's second) she died by jumping into an interdimensional portal that would have destroyed the world so that Dawn would survive. She was later brought back from the dead by a spell performed by Willow, Xander, Anya, and Tara.
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