Ilex Forest
There is a shrine for the guardian of the forest, Celebi. In Pokémon Crystal, if Kurt has already examined the GS Ball, placing it in the shrine will cause a level 30 Celebi to face the player. Though the GS Ball was never distributed as an event item outside of Japan, the Virtual Console release of Crystal allows it to be obtained in all regions through normal gameplay.
source GS Ball
Acquisition Goldenrod City Pokémon Communication Center/Pokémon Center
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