Planescape: Across the Multiverse
The laws of magic are not entirely universal across the Outer Planes. When on one of the outer planes, a character will require what’s known as a Spell Key to cast magic normally. Casting can be done without a spell key, but requires a great deal of additional effort.
For each power point spent, the caster must add +1 to the target number for a sucessful spellcasting check.