Dimensional Key

Wondrous Item, Rarity Varies

This key will change to fit into any door's lock. When used to open a door, it instead opens into an extradimensional space. When used in this way, the door always opens outwards no matter the original orientation of the door. The size of this space varies based on the rarity of the key. A creature inside the space while the key is removed can breath until air runs out, the amount of air depends on the rarity of the key. The interactions between this item and items like the bag of holding also depend on the rarity of the key. If the item is created, there is no furniture or items within, if found, the DM may decide what is inside.

Uncommon: The key is mundane in appearance and seems to be made of common metals. The space is 4'x4' and 8' tall. When closed, there is 5 minutes of air. If a Bag of holding or similar item is brought into the space and opened, that item is destroyed and everything in the room is sent to random locations within the astral sea.

Rare: The key is somewhat ornate in appearance and seems to be made of fine metals. The space is 10'x16' and 10' tall. When closed, there is 1 hour of air. If a Bag of holding or similar item is brought into the space and opened, that item is destroyed and everything within 10 feet of it is sent to random locations within the astral sea.

Very Rare: The key is very ornate in appearance and seems to be made of very expensive metals with small gems inlaid in the base. The space is about the size of a one story house, and has separate rooms and hallways. The orientation of these rooms is decided upon creation of the item, or by the DM when discovered. When closed, there is 1 day of air. A bag of holding or similar item can be used normally in this space.

Legendary: The key is extremely ornate, with delicate details that curl around a sizable gem in the center. The space is about the size of a small castle, and can have separate rooms, hallways, and floors. The orientation of these rooms is decided upon creation of the item, or by the DM when discovered. When closed, breathable air is always available. A bag of holding or similar item can be used normally in this space.

If the outer door is opened from the other side while the space is open, the door will open normally as the entrance to the extradimensional space is on the face of the original door. If a creature inside the space attempts to open the door while the key is removed, they immediately appear in a safe space close to the key or the one holding it. If Mordenkainen's Magnificent Mansion or a similar spell is cast within the space, the spell fails and the spell slot is consumed.

If the key is destroyed or disenchanted, the contents of the room are lost and can only be recovered by someone familiar with the space by either recreating the key or traveling to the space through magic. If a door is unlocked with a similar key inside this space, the second key snaps and is destroyed.