Kronos - Immortality Clause

Maybe you’re thinking, Wait a minute. If the kid is immortal, why is Rhea worried about hurting her? But, see, immortals can be hurt badly, crippled, or mutilated. An injury might not kill them, but they also don’t always heal from damage. They just stay crippled forever. You’ll see some examples of that later on. Rhea wasn’t about to cut open Kronos and risk chopping up her baby, because being in pieces is no way to live, especially when you live forever.

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Greek Gods, The Golden Age of Cannibalism