Magical Effects on the Planes (PWH)

__TOC__
The first thing a planewalking wizard (and to a lesser extent a priest) has to realize is that her spells are not absolute. They are malleable and fallible, dependent on the plane where they're cast as well as a number of other related factors. Each plane maintains its own rules about how magic works. It's a lot for a cutter to remember, but a body should know the dark of her trade and livelihood, right?
All planar wizards and priests have access to the following information. It's also available to primes who inquire in Sigil or other large burgs like the gate-towns of the Outlands. As always, a basher wanting the knowledge has to pay a little music.
==General Conditions==
Spellcasting on the planes relies on three basic principles (Rule of Threes, of course!): Who's native to the plane, where the plane is in relationship to other planes, and (in the case of realms) what conditions have been imposed by the power who rules there. These principles, explained below, apply to both wizards and priests. Rules specific to each class follow.
===1. Planar Natives===
Just like a prime is a native of the Prime Material Plane, everything's got a home plane. Regardless of where a body stands, he's still a native of his plane of origin. Some call it the Law of Origins, or even the Law of the Home-Born.
A basher on his home plane isn't extraplanar. Pretty simple, right? A body can't banish a tanar'ri from its home plane of the Abyss. Therefore, spells like banishment, binding, dispel evil, holy word, and protection from evil don't work against creatures native to the plane they're cast on. (In the case of protection from evil, this restriction only affects the aspect of the spell that wards against extraplanar creatures, and not the spell's other general protective qualities.) Further, a gate spell cannot summon a creature from the plane the caster stands on. (The innate ability of a fiend to gate its fellows isn't hindered by this restriction. Why? That's just the way it works.)
The Law of Origins is even more effective when it concerns petitioners, since petitioners can be considered to have been "born" on the plane they've taken form on. In addition to the above notes, healing and necromantic spells of any kind—beneficial or harmful—have no effect on these folks.
Perhaps because of their origin on the Prime Material Plane (which touches all planes through the Astral and Ethereal), these principles apply to primes as well, no matter where they stand; even on Gehenna or the plane of Lightning, primes aren't considered extraplanar and aren't affected by such spells.
===2. Summonings===
On the other hand, on all planes but the Prime, spells other than gate that conjure or summon extraplanar creatures can only call up monsters native to the plane on which the spell is cast, or on a directly adjoining plane. For example, monster summoning spells on the Gray Waste capture natives to that plane—diaaka, night hags, hordlings, and other horrible evil creatures—and can also snatch beasts from Gehenna, Carceri, the Outlands, and the Astral (though that doesn't really expand the caster's prospects much). On Arcadia, a basher shouldn't hope to summon similar creatures—instead, his spell'll grab natives of Arcadia or its surrounding planes.
The rules concerning elemental summonings change because of these principles, too. (They're also affected by the axiom of Dimensional Relations, as described below, but the end result is the same.) When a priest—or a mage armed with the proper spell key—conjures an elemental on the Outer Planes, she draws upon the forces on whatever plane she's on to create the elemental. Thus, when a priest on Elysium casts conjure fire elemental, what really appears is an Elysium-born spirit animating native elements to resemble a fire elemental—but it's not a true elemental in the strictest sense. In ability and power, however, it acts like the appropriate creature—with one exception. The "elemental" takes on some of the aspects of the plane from which it's created.
The Elysium fire elemental is sincerely and strictly neutral good in alignment and does nothing to contradict that good essence. Likewise, a similar creature summoned on Gehenna has an evil essence, and attempts to trick and subvert the caster in much the way a summoned fiend would.
===3. Realms===
The powers dictate the magical conditions in their own realms, imposing penalties and benefits as they see fit. Conditions of the surrounding plane may be completely absent, or they may be twice as potent. Magic can function at peak efficiency, or lose its power altogether. These conditions utterly depend on the nature of the power who controls the realm, and may not be overcome save by the power's own whim.