Purchasing this item will give you 1 Legendary Crate Key in-game. Use it at /warp Crates to get 1 randomly-selected item. That item being a random Legendary Pixelmon, a random Ultra Beast, a random SHINY Legendary, a random SHINY Ultra Beast, or a Fantasy Crate Key (used to get a random Textured Legendary Pixelmon)*.
*You can view a list of current crate contents in-game at /warp Crates by punching the crate.
(1) - You are NOT guaranteed to get any "highly valuable" items from Legendary Crates.
(2) - By buying these keys, you are agreeing that the contents of the Crate are randomized, and that this is essentially gambling for in-game items. You will not be able to receive a refund under any circumstances.