Beedle probably took the idea from the Sorcerer’s Stone; you know, instead of a stone to make you immortal, a stone to reverse death.
Bellatrix and Lucius Malfoy threw others behind them in their race for the door, and again and again his wand fell, and those who were left were slain, all of them, for bringing him this news, for hearing about the golden cup — Alone amongst the dead he stomped up and down, and they passed before him in vision: his treasures, his safeguards, his anchors to immortality — the diary was destroyed and the cup was stolen.
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Zeus and the other gods were supposed to be immortal.