The Circlet of Netheril, by itself, is a component in a larger item. Xen wants you to talk to your 'servant' about it. If you haven't gotten to ToB yet, don't concern yourself with it until you get there.
As for the demons in the antimagic zone, I'd stand by the door for a while and let them tear each other apart. Then you can just waltz in and wipe out whatever's left. Stay out of sight, or they'll unite against you, the common foe.
|