[quote=@Tomaru]
<Snipped quote by AlternateMan>

What were you planning on doing with it?
[/quote]

To give it functions like summoning a captured youkai/mythical creature(balanced so they won't be hella op, of course) or borrowing their powers. It'd be something like a spellbook of summoning spells that gets better as the user 'captures' a strongef youkai.
The magical ability given to the character would be nothing else than the permission to use the functions of the crystal ball.