Solipsism

This unusual spell is similar to Phantasmal Force and other illusion magic, except that the priest who casts the spell is the only creature who automatically believes the results of the spell.
The spell creates the illusion of any object, creature, or force, as long as it is within the boundaries of the spell's area of effect. The illusion is visual and tactile (that is, it can be seen and felt), but no other sensory stimuli are created.
Solipsism is the opposite of normal illusions in that anyone other than the caster must make an active effort to believe (rather than disbelieve) the illusion. Characters trying to believe the reality of a solipsistic illusion must make a saving throw vs. breath weapon, modified by the magical defense adjustment for Wisdom. A successful save means that the character believes the illusion and it is part of reality for him. A failed save means that the character cannot convince himself of the illusion's reality, and the illusion has no effect on him. A character can make a single attempt to believe each round.
Unlike true illusions, the image created by this spell does more than just duplicate reality. The image formed is real for those who believe in it. The illusion has all the normal properties that its form and function allow. Thus, a solipsistic bridge spanning a chasm could be crossed by the priest and those who believed. All others would see the priest apparently walking out onto nothingnesss. Likewise, a solipsistic giant would cause real damage to those who believed it.
The illusion remains in effect for as long as the priest continues to concentrate on it, until the priest is struck in combat, or until he is rendered unconscious. The level of concentration required is not extreme; the priest can move normally and may engage in combat, but is unable to cast any spell while maintaining a solipsistic illusion.
Solipsism can create only illusions that are external to the priest. Thus, the priest cannot create an illusion that he is the size of a giant, is unwounded, or has sprouted wings.
The priest must swallow the bit of lotus blossom while casting the spell.

[Tome of Magic]

Sphere: Thought
(Alteration)
Level: 4

Range: 10 yards/level
Components: V, S, M
Duration: Special
Casting Time: 1 round
Area of Effect: 100 sq. ft. +100 sq. ft./level
Saving Throw: Special

Material Components:
a lotus blossom and a bit of fleece