According to the accounts of demons said to be familiar with this realm, it is lonely and desolate, filled with horrific scenes of darkness and shadow, fear and surprise. The resultant tension constantly cycles to a crescendo of madness and horror. Consequently, this realm is the least populated of all the realms of Hell, as few demons can withstand its unrelenting torment. The Armored Destroyers were an exception, and prior to the Dark Exile, "crawled like locusts" over the realm.
An exception to this was the Nephalem, who confronted Diablo at the Crystal Arch. After engaging in battle, Diablo dragged his foe(s) down into the Realm of Terror, where they were forced to battle duplicates of theirself/themselves. The Nephalem prevailed, and returned to the Arch, where the Lord of Terror was later defeated.
The Realm of Terror briefly appears as a zone in Act IV of Diablo III, serving as the arena for the second phase of the boss fight against Diablo. The player(s) have to kill Shadow of Diablo while also avoiding/slaying the Shadow Clones he summons. After this, the third phase of the boss fight begins.
The area is quite large, with Diablo's runes inscribed on the rocks. It has multiple impassable pits, and the vision range is limited due to swirling fog.
Realm of Evil (Infernal Machine event location at level 70) is absolutely identical to Realm of Shadow, except for it is not covered in fog. In addition, one of the Realms of Evil in this event is named Realm of Terror.