Need help finding location - geode

[isoChron]

I remember some official WotC stuff describing a giant geode which can be entered through a portal and there are more portals to single rooms inside. Was home of a dead wizard and such. But I can't find it anymore. I'm pretty sure it was from WotC. Not sure if it was a free adventure or some stuff inside a book (or both).
Anyone know what I'm talking about ?

 

[Huw]

Lost Empires of Faerun. It's an adventure site called Sanjar's Tower.

 

[isoChron]

Yep, that's it. Thanks for help.