Summon-Slave Crystal

This thumb-sized crystalline skull is carved with necromantic runes. Once per day, a spellcaster can grasp the stone while casting a summoning spell. When the spellcasting is finished, the caster may immediately attempt to possess the summoned creature, as if using magic jar, except as a free action. The creature does not get a saving throw to resist, though spell resistance still applies. If successful, the caster possesses the creatures' body until the duration of the summoning spell ends, at which time his mind returns to his body, the creature's mind returns to its body, and the creature disappears.