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Luster keys contain a chance at getting a shiny pokemon. Once you got your key visit /warp crates
Mega keys contain a chance at getting a shiny pokemon with its corresponding megastone.
Sacred keys contain a chance at getting a random legendary or Ultra Beast Pokemon from the Sacred Crate.
Cosmic keys contain a chance at getting a random shiny legendary or Ultra Beast Pokemon from the Cosmic Crate.
You can see all the things you can get from the PokeKey by clicking on the PokeCrate in /warp crates.
You can see all the things you can get from the GreatKey by clicking on the GreatCrate in /warp crates.
You can see all the things you can get from the UltraKey by clicking on the UltraCrate in /warp crates.
You can see all the things you can get from the MasterKey by clicking on the MasterCrate in /warp crates.