Mirror Realm

You transform a mirror into a magical doorway to an extradimensional realm. You and any creatures you designate when you cast the spell can move through the doorway into the realm beyond. For the spell's duration, the mirror remains anchored in the plane of origin, where it can't be broken or otherwise damaged by any mundane means. No creatures other than those you designate can pass through the mirror or see into the mirror realm.

The realm within the mirror is an exact reflection of the location you left. The temperature is comfortable, and any environmental threats (lava, poisonous gas, or similar) are inert, harmless facsimiles of the real thing. Likewise, magic items reflected in the mirror realm have no magical properties, but those carried into it work normally. Food, drink, and other beneficial items within the mirror realm (reflections of originals in the real world) function as normal, real items; food can be eaten, wine can be drunk, and so on. Only items that were reflected in the mirror at the moment the spell was cast exist inside the mirror realm. Items placed in front of the mirror afterward don't appear in the mirror realm, and creatures never do unless they are allowed in by you. Items found in the mirror realm dissolve into nothingness when they leave it, but the effects of food and drink remain.

Sound passes through the mirror in both directions. Creatures in the mirror realm can see what's happening in the world, but creatures in the world see only what the mirror reflects. Objects can cross the mirror boundary only while worn or carried by a creature, and spells can't cross it at all. You can't stand in the mirror realm and shoot arrows or cast spells at targets in the world or vice versa.

The boundaries of the mirror realm are the same as the room or location in the plane of origin, but the mirror realm can't exceed 50,000 square feet (for simplicity, imagine 50 cubes, each cube being 10 feet on a side). If the original space is larger than this, such as an open field or a forest, the boundary is demarcated with an impenetrable, gray fog.

Any creature still inside the mirror realm when the spell ends is expelled through the mirror into the nearest empty space.

If this spell is cast in the same spot every day for a year, it becomes permanent. Once permanent, the mirror can't be moved or destroyed through mundane means. You can allow new creatures into the mirror realm (and disallow previous creatures) by recasting the spell within range of the permanent mirror. Casting the spell elsewhere doesn't affect the creation or the existence of a permanent mirror realm; a determined spellcaster could have multiple permanent mirror realms to use as storage spaces, hiding spots, and spy vantages.