No you can enter it, it's the Twisted Rune encounter.
MAJOR SPOILER
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There's a vamp which drains 4 levels a hit, a beholder, a fighter (the weakest character by far), a high level mage and an ultra high level mage in there. It don't matter if you stand off or close for combat either way you're well and tuly buggered if you're not well prepared. (And quite competant)
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