If guardians can be revived by their ghosts, then why didn't jaren ward be revived? Guardians are immortal.
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If you kill a guardians ghost, there is no ghost to revive him.
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Dredgen yor's thorn drained his light beyond revival
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I'm guessing Yor kept his ghost away from him, or Thorn had completely drained his Light until there was nothing left. It's the same way the Guardian on the Moon died and our ghost couldn't resurrect him.