This pair of keys looks to be made of Muspelia, a type of Adamantine that has been hardened by the heat of the Elemental Plane of Fire. These keys come in pairs - one of them larger with a Padlock for a handle, and the other, smaller key linked by way of an ethereal chain. 

As an action, you are able to place this key into any door with a keyhole, and turn it - the door it is stuck within locks as if it had the correct key used on it. You can unlock a door that has been locked in this way as a bonus action by turning the key in the opposite direction.

Double-Lock. As an action while the key is locking a door, you may slip the smaller key attached to the larger one into its padlock. While "Double-Locked" in this way, the door gains the properties of a Wall of Force, and remains like this while the smaller of the two keys is stuck in the larger key's padlock. The larger key cannot be removed from the door while "Double-Locked".

Notes: Social, Utility, Shapechanging, Warding, transmutation