The first time I opened it, it of course blew up and knocked out half of my party. When I actually got inside, I just heard that typical "teleport" sound and I already knew that something will be very wrong... When I got away from there, only my knight was alive with couple of HP.
(Since I'm going so slowly just wandering around, the whole area actually respawned once, so I got to open it for the second time. And after being teleported, I even cleared up all the gogs.)
By the way, I just hired a locksmith to mop up the Haunted House (I cleared all monsters, but I left the chests behind) and go through all Barrows (incidentally, I made a careful graph of the connections between the Barrows which will probably turn utterly useless). And in one of those cabinets, there was a godsend (even without mephistoing)... a belt with Disarm + 12. So I hope that "effective disarming" 31 (or maybe 63? Not sure if it adds to the skill itself (and gets tripled by the Master bonus), or if it adds only to the result) solves the problem for a while.
And I just thought that Celeste was another wizard city somewhere up in the clouds... I wanted to get there mostly to play Arcomage . Now when you say that I need to satisfy some conditions... maybe I already know where the teleporter is. Is it the weird blue tripod-like thing that is right next to one of the teleporters from the "Main Square"? I was clicking it in all possible ways (and I even jumped through the loop on the top!) and nothing ever happened...
As for the battles: sadly, the combat in the MM games isn't really appealing to me. It mostly reduces to "click one enemy until it dies, then click the next enemy, but if it's you who's being whacked, run away like hell". Though I must say that MM7 adds quite a lot of meat like enemies who explode on death or enemies who can cast buffs or summon more monsters (in MM6 they mostly use some destructive spell which you can often hardly tell from their standard ranged attack). And I quite didn't like castle Darkmoor. (I would probably hate it 75% less if Dispel Magic didn't affect Torch Light and Wizard Eye, though )