The Kingdom of Shadow is an uberlevel whose entrance is hidden carefully in the Ruins of Ureh, itself only accessible through the Vizjerei Temple. There's a banner flag on a pole near the arrival point to the Ruins of Ureh; the entrance to the Kingdom of Shadow lies in the general direction from the arrival point to that flag. The distinctive feature of this area is that almost all monsters there are untargetable and unkillable, except the occasional Darkwarden or Punishment Turret.
The mythical city of Ureh, where Diablo took the form of an angel and deceived the city wizards, tricking them into casting a spell that doomed the city. Ureh and everyone within are now trapped between the mortal plane and the Burning Hells, appearing in this world only once every two thousand years, when the shadow of Mount Nymyr falls upon the ruins. Be there.