that's what happen when you get stuck with common sense in Gensokyo... you think you're doing something so good but end up realizing that someone else already beat you to it over a millenia ago... XD
Enlong said: Well, even Eirin couldn't have made it alone. She just happens to be paired with someone who has the power of Eternity.
Kaguya became immortal from consuming Eirin's Hourai elixir. And she was exiled from the Lunar Capital for it. Eirin is the only one responsible for it.
IIRC Kaguya had "power over eternity" whatever that means, before she was immortal, and that power was a key part in creating the elixir in the first place.