So, thinking a bit about how this could work, I came up with a quick write-up of a potential spirit as a sort-of magic item:
Suruq'esh of the Hundred Eyes
Description: Suruq'esh is a multidimensional spirit whose gifts include the ability to move across the dimensions and to seek out visions for its master. It appears as a formless mass of translucent grey mist in which float many ice-blue eyes that blink in and out of existence, though it may turn invisible at will. Suruq'esh values knowledge and secrets, and speaks in a hoarse whisper that causes chills to all who hear it.
Summoning: Suruq'esh may be summoned by means of the rituals detailed in the Tome of the Azure Eye, an ancient text of which only one copy remains. The pages of the book are stamped bronze plates bound in leather, and inscribed in ancient Aquilan. The ritual of summoning takes five hours and demands the sacrifice to Suruq'esh of a rare book.
Binding: Suruq'esh may be bound by a contract, and will serve willingly any master who speaks the binding spell in the Tome of the Azure Eye, though it may be bound to only one master at a time.
Service: Suruq'esh may be summoned by its master to perform the following tasks:
• Use the Crystal Ball spell at will.
• Detect Invisibility three times per day.
• Wizard Eye once per day
However, binding Suruq'esh also affects the caster. The caster will gain ice-blue eyes like the spirit, and his skin will become cold to the touch.
Suruq'esh has an Intelligence of 14, an Ego of 12, and speaks Common and Ancient Aquilan. If at any point the master's Intelligence + Wisdom + level is less than the spirit's combined Intelligence and Ego, Suruq'esh's service is ended, and the spirit may possess the master.