Historically, alter reality is identical to a limited wish spell. That deprives the illusionist of a real signature pinnacle spell, and treads on the magic-user's toes. And it's boring. A more recent version turned the spell into a short duration buff for any illusions in the area, which again sells the illusionist short. And it's boring. If there's one thing an illusion shouldn't be, it's boring.
This version of the spell sets up a comic-book style rewrite of reality - everything has changed, only the heroes remember "reality", and only they can change it back. Alternately, the PCs can use it, but the advantages are fleeting. It is both broader in scope and more ephemeral than wish.
Level: Illusionist, 9th Level
Range: 1 mile radius per level
Duration: See below
Upon casting this spell, a silvery blast of light radiates outward from the illusionist to the limit of the spell. Within the spell’s range, reality reshapes itself according to the caster’s desires. The effect reaches backwards in time up to one day, so battles that were lost might be won, the slain returned to life, and so forth.
All those within the area of the spell’s effect, however, have a persistent sense of displacement and déjà vu, the severity depending on how dramatically their reality changed. A person returned from the dead has a constant feeling of hopelessness and apathy, for instance, while someone else might feel displaced, or that they've forgotten something important. Changed creatures and items taken beyond the area of effect maintain their changed state (but see below).
Characters and creatures of half the caster’s level and higher get a saving throw; if successful they remember “reality”, but this does not undo any circumstances currently affecting them.
The altered reality remains in effect until the caster dies, he leaves the area of effect, or dispels the spell. Once that happens reality reasserts itself. The dead become dead again, and their illusionary “living” forms disappear. A character that was killed by the altered reality awakens, alive, at the site of their “death”. Other effects are up to the Referee, but in general things should revert without serious physical consequences. After dispelling or exiting an alter reality spell, the caster is mentally exhausted and cannot cast spells for 1d6 days.