The Cave is located in Terror from Beyond–under the bridge towards the second boss. When the cave is entered on 16-man HM, the following message is displayed: “You feel a strange and chilling presence.”
When you go in with the Dread Mask and use the Dreadful Amulet (both drop from Dreadtooth–the latter only dropping when he is Nightmarish), a boss called “Dreadful Entity” spawns. He does A LOT of damage and spawns electric puddles and he has a ridiculous amount of health.
He can be pulled pretty far from being spawned in the cave and has a really cruel knockback 🙂
He drops an item called the “Dreadful Orb”.