Ethereal Key

A fancy-looking silver key. The bow of the key has a hexagon gem embedded into it, which has a rich blue color emitting a faint light. When inspected closely, it seems as if the inside of the gem consists of a thick mist. On the shaft, three sets of wings are attached and made from the same material as the key. Each pair is different, one of an angel, one of a fae, and one of a dragon.
The key functions as an additional spell slot, of a tier equal to its carrier's proficiency modifier. When casting a spell, instead of spending a spell slot, you can use the key. Doing this causes the gem embedded in the bow to turn dull, making the key useless. It takes two hours of concentrating on the key, slowly embuing it with magic, to restore the gemstone.
Alternatively, a creature can use the key to transport itself to a subspace of the ethereal plane. As a bonus action, the creature must hold out the key and turn it, as if unlocking a door. After doing this, it is instantly transported. The creature can stay in the ethereal realm as long as they like, however, due to the warping of time, the creature will return to the primal realm at the end of its next turn. Creatures in this subspace cannot benefit from a long or short rest. Using the key in this manner will also dull the gemstone in its bow. 
The subspace of the ethereal plane consists of a small cobbled road floating in empty space. All around, constellations both known and unknown to mankind can be seen scattered around the space. At one end of the cobbled road stands an arch. When walking through it, the creature will return to the primal realm, in the exact spot they left. On the other side is a small square with a well in the center. The well is filled with a rich blue liquid the same color as the gem of the key. This well is a source of innate magical power. When drunken, the pure magic in the well can have different effects depending on the creature. For example, a sorcerer is able to restore a number of sorcery points equal to their proficiency level. Additionally, a creature drinking the magic will be healed for 2 times their level and they have advantage on rolls related to spell effects, concentration saving throws, and any intelligence-based check for 1 hour.
A creature must attune to the key to use it. This process is likely to fail for most adventurers as only high-level magic casters can attune with the key. An exception to this, however, are sorcerers, whose ancestry allows them to resonate with the key.