The reward for completing all 18 arenas of the Colosseum within one hour. It opens the Colosseum Vault which holds Black Marrow Reliquary grade loot plus a random ring (Red Ring, Blue Ring, Yellow Ring, White Ring, or Green Ring).
Note: While the key is attuned and bonded you can can open to the chest and then let someone else loot it hence they are tradeable in a way.
