[Much of the material here was gakked from the Stronghold Builder’s Guidebook and then modified.]
A portal is simply a long-term stable teleportation effect that safely whisks its user to a predetermined place. In the Polychotomy, portals are a fact of everyday life for anyone who moves between shards. Even powerful spellcasters with access to gate and greater teleport prefer the convenience of portals for their quotidian transportation needs.
Kinds and Qualities of Portals
There are two kinds of portals commonly found in the Polychotomy, called Fixed and Unfixed. Fixed portals tend to be significantly more stable than unfixed ones, and each is adaptable in some ways that the other is not.
Every time a new connection is forged between shards (whether through the creation of a new portal, a spell effect of some sort, a magic item that creates an extradimensional space, or other reasons), all shards are forced to adapt to the new configuration. The results of this reconfiguration are different for the two kinds of portals. An unfixed portal will tend to drift with time – for every 30 days it operates, each end of the portal has a 5% chance of moving up to 1 mile from its original location. This can be offset with the stabilize portal spell. Fixed portals are much more stable, having only a 5% chance per century of destabilizing, but when a fixed portal destabilizes, the portal disappears completely. Moving an extradimensional effect (such as a bag of holding, or portable hole) through a fixed portal also has a 5% chance of destabilizing the portal as soon as the extradimensional effect has passed through. (Note: for these purposes, an unattached Ab is considered extradimensional, assuming you have some way of moving it.) Casting stabilize portal can also offset these chances.
Portals share some common qualities. All portals are two-dimensional areas, usually a circle with a radius of up to 10 feet, but sometimes square, rectangular, or another shape. (Larger portals are more expensive, but necessary for creatures of greater than Huge size.) The portal itself is intangible and invisible.
Portals are usually constructed in pairs or networks. A non-paired portal is essentially a one-way trip, rather less useful in the Polychotomy. As a result, a matching return portal is generally constructed going the other direction. Occasionally a portal will be attuned to several different destinations. Once created, a fixed portal cannot be moved. Unfixed portals can be moved using a portal case.
An archway or frame of some kind usually marks a fixed portal’s location so it can be found when needed and so that creatures don’t blunder into it accidentally. Detect magic can reveal a portal’s magic aura. If the portal is currently functioning (ready to transport creatures), it has a strong aura. If the portal is not currently able to transport creatures (usually because it has a limited number of uses, and they are currently exhausted), it has a weak magic aura. Strong or weak, a portal radiates transmutation magic.
Even a character capable of locating traps can’t find a portal with the Search skill.
Creatures who touch or pass through the area of a fixed portal are instantly teleported to the locale the portal’s builder specified. (the teleportation efect is similar to greater teleport cast by a 17th-level caster.) It is not possible to poke one’s head through a fixed portal to see what’s on the other side. Unfixed portals allow for a person to stick only a portion of their body through. A portal can only transport creatures that can fit through the portal’s physical dimensions.
If a solid object blocks the destination, the portal does not function. Creatures, however, do not block portals. If a creature already occupies the area where a portal leads, the user is instead transported to a suitable location as close as possible to the original destination. A suitable location has a surface strong enough to support the user and enough space to hold the user.
Unattended objects cannot pass through a fixed portal. For example, a character can carry any number of arrows through a portal, but he cannot fire an arrow through a fixed portal. An unattended object that hits a fixed portal simply bounces off. Unattended solid objects can pass through an unfixed portal.
Unless the builder has preset some limit, any number of creatures can pass through a portal each round. A creature using a portal can take along up to 850 pounds of gear through a fixed portal, up to 680 pounds of gear through an unfixed portal. Fixed portals can be constructed with larger load limits – construction costs (in both gold and xp) are pro-rated accordingly. In this case, gear is anything a creature carries or touches. If two or more creatures touch the same piece of equipment, it counts against both creatures’ weight limits.
Portal builders often restrict access to their creations by setting conditions for their use. Special conditions for triggering a fixed portal can be based on the possession of a portal key, the creature’s name, identity, or phyle, but otherwise must be based on observable actions or qualities. Intangibles such as level, class, Hit Dice, or hit points don’t qualify.
Keyed portals may be created at no extra cost. The key must be designated during the creation of the device and cannot be changed after that.
A keyed portal remains active for 1 full round. Any creature that touches the activated portal in the same round also can use the portal, even if such creatures don’t have a key themselves.
Many portal keys are rare and unusual objects that the creature using the portal must carry. Some portals are keyed to work only at a particular time. Spells can serve as portal keys, as can the channeling of positive or negative energy. When the portal is the target of the specified spell or within the spell’s area or touched by its effect, the spell is absorbed and the portal is activated. Any form of the spell works to activate the portal, including spell-like effects of creatures or magic items and spells from scrolls.
A portal cannot be destroyed by physical means or by spell effects that destroy objects (such as disintegrate). A successful targeted dispel magic (DC 28) causes a portal to become nonfunctional for 1d4 rounds. Mordenkainen’s disjunction destroys a portal unless it makes a Will save (a portal’s Will save bonus is +10).
Portals can’t be disabled with Disable Device. Carrying a large quantity of extradimensional effects (e.g. bags of holding) through a fixed portal has a high statistical likelihood of destabilizing it, but is not a sure thing.
Building a Portal
Any character of at least 17th level can build a portal if she knows the Create Portal feat and either the teleportation circle (for intra-shard portals) or gate (for inter-shard portals) spell. See below for portal prices.
The portal can lead to any locale the builder has personally visited at least once.
The portal fails if the builder chooses a destination that cannot safely hold her (such as inside a solid object or into thin air). The portal also fails if the destination is a locale which is specifically sealed off from teleportation.
The prices and construction times noted are for portals that operate constantly, transporting anyone who passes through them at any time. If the portal can be used less than five times per day, the base costs are reduced. Multiply the market price by the number of uses per day (minimum 1, even if the portal cannot be used once a day), then divide by 5. This gives you the new market price of the limited-use portal. Each activation of a limited-use portal lasts 1 round. Once activated, a limited-use portal can transport as many creatures as can touch it in that round.
Fixed Portal: This continuously active portal allows one-way travel only. It covers an area with radius of anything from 6 inches (about 1 square foot) up to 10 feet (about 300 square feet).
A larger portal adds 100% to the market price for each extra 300 square feet or fraction of 300 square feet.
Caster Level: 17th; Prerequisites: Craft Portal, gate or greater teleport, caster level 17; Market Price: 100,000 gp.
Fixed Portal, Two-Way: This construction is actually a matching pair of portals, one at each end of the “trip”. It is otherwise identical to a standard portal.
Caster Level: 17th; Prerequisites: Craft Portal, gate, or teleport without error, caster level 17; Market Price: 150,000 gp.
Unfixed Portals cost 80% of fixed portals (80,000 gp for one-way, 120,000 for two-way). An unfixed portal can be turned into a fixed portal for 25% of the cost of a fixed portal (25,000 gp for one-way, 37,500 gp for two-way).