This is the home of the notorious Sin. It was built when Atra wasn't dark elf territory (a looooong time ago, but remember Sin is ageless); her adoptive parents lived here until she murdered them on her eighth birthday. It collapsed a time later, creating a tunnel, and Sin lets basically no one in and will recieve no person. But maybe if you're strong, as strong as her, she'll see you as an equal (very slim chance of that) and let you in...