Did you say demons? That's a Warlock's bread and butter hehe. No, really... I'm sure it'll be tough, but 'locks do have a crap load of skills/talents to deal with demons. Banish makes them inactive for a minute (though also invincible), enslave demon captures a demon and makes it our pet for 5 mins (and we get to use their specials too if they have any), there is some spell called sense demons or something that puts all the demons on the radar (sounds lame, but who know, maybe it'll help navigate in this place), and our voidwalker consumes demons to fill his mana up (not sure if this actually kills demons or not).
I'm not too sure exactly what it'll take to open that thing, but I'm pretty sure it'll require at least a handfull of level 60 Warlocks, not to mention a good raid party to help them get there.