Greater Rift Keys (GRK) are a Legendary variant of the Rift Keystone Fragment. They open a Greater Rift when applied to Nephalem Obelisk. Rank of the Greater Rift is equal to the level of the GRK used, and this number is displayed both in the description and on the item's icon.
Getting a GRK requires player to obtain the Keystone of Trials from any Rift Guardian in a regular Nephalem Rift first, then complete the Realm of Trials. Doing so will grant a GRK with rank proportional to the number of waves cleared.
The GRK is not lost if player beats the Greater Rift in time: Urshi may upgrade it to higher level instead of upgrading Legendary Gems. However, choosing to upgrade gems, or failing the Rift, will destroy the GRK, forcing the player to find a new one.
To participate in the Greater Rift run, every player must contribute a GRK of equal or higher rank compared to the first one used.
Greater Rift Keys are not unique; the player may have as many as they can obtain at once, and must choose which of them to invoke and upgrade every time.