Ways to enter the planes:
- Portals, conduits and gates are possible entrances.
- Portals: Players can cast a portal and perhaps randomly arrive there. Other may be a portal that is through a "door" that appears random or placed somewhere but a key of some type is needed to open it. This portal disappears for a time after being entered.
- Conduits-the colors pools in the Astral plane would be a conduit. A whirpool that appears, etc.
- Gate-perhaps a "wild magic" of the gate spell?

Planes

Mirror

A hall of mirrors that leads to various planes.
At times one of these works as a real mirror that duplicates the player but reverses the alignment and makes the "mob" hunt and attack the player.