Kill the Demon of the Orb, then speak with Tabetha.
Ah, a summoner's orb--this is where the taint must be coming from. We must exorcise the demon within it if you're to use it. That I can do easily.
You're the one who has the hard job. You must kill the demon once I release it.
And you'll have to be quick. If you take too long then he'll return to the orb and we'll have to start over!
So get yourself ready, [player]. Prepare your strongest spells and keep your potions handy, and go get a friend if you have one. Because this isn't going to be pretty.