Soul of Arkrax:

The wielder of the Soul of Arkrax may choose to cast spells through it. Doing so decreases their cost by 2, to a minimum of 2. There is a 10% chance each time a spell is cast through this artifact that Arkrax will secretly seize control of it, and may use the same amount of magic points to cast any spell available, potentially casting the same spell. This chance increases by 2% each time this chance fails to trigger; resetting when it is triggered. Given that it is secret, the spell will always appear to have been cast by the original caster. "You think that you're rid of me? Hmph. You'll see just how patient a dragon of my age can be..."