The portal functions only at particular times: during a full moon on the Material Plane, or every ten days, or when the stars are in a particular position. Once it opens, such a portal remains open for a limited time, such as for three days following the full moon, or for an hour, or for 1d4 + 1 rounds.
“Portal” is a general term for a stationary interplanar connection that links a specific location on one plane to a specific location on another. Some portals function like doorways, appearing as a clear window or a fog-shrouded passage, and interplanar travel is as simple as stepping through the doorway. Other portals are locations — circles of standing stones, soaring towers, sailing ships, or even whole towns — that exist in multiple planes at once or flicker from one plane to another. Some are vortices, joining an Elemental Plane with a very similar location on the Material Plane, such as the heart of a volcano (leading to the Plane of Fire) or the depths of the ocean (to the Plane of Water).
Passing through a planar portal can be the simplest way to travel from the Material Plane to a desired location on another plane. Most of the time, though, a portal presents an adventure in itself.
First, the adventurers must find a portal that leads where they want to go. Most portals exist in distant locations, and a portal’s location often has thematic similarities to the plane it leads to. For example, a portal to the heavenly mountain of Celestia might be located on a mountain peak.
Second, portals often have guardians charged with ensuring that undesirable people don’t pass through. Depending on the portal’s destination, “undesirable people” might include evil characters, good characters, cowards, thieves, anyone wearing a robe, or any mortal creature. A portal’s guardian is typically a powerful magical creature, such as a genie, sphinx, titan, or native of the portal’s destination plane.
Finally, most portals don’t stand open all the time, but open only in particular situations or when a certain requirement is met. A portal can have any conceivable requirement, but the following are the most common: