You are bonded to a World Soul, a consciousness that was once the entire awareness of a plane of existence. It's original plane of existence either no longer exists, or it has somehow become separated from it. Either way, this consciousness has found a new host inside yourself. This offers you a truly vast and incomprehensible amount of mana with which to cast your spells. The consciousness has an awareness that it can use to guide you, and improve your spells beyond what you would normally be capable of. You can use this to make spells simply more powerful than they otherwise would be, regardless of the mana at your disposal, or find ways to extend or improve them in other ways. This World Soul allows it's bearer to planeswalk as well, though it only makes you a post-mending planeswalker.
The World Soul will initially be relatively benevolent, and will be on your side for the most part. Of course, if you end up being someone who doesn't use their powers at all, it might feel that there's a more worthy host out there for it...