The Void. It's...another universe, sort-of, that has magic, but also way less energy than Earth or Equestria do. It's somewhat implied that it exists in a higher-entropy state than our world does. Some of the later PaP stories explore into it a bit.

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how old was Charybdis?

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Not stated. If you really want to find out, join the Discord and ask Starscribe directly: https://discord.gg/UCe3EnvP