The portal was orange, with silver-black viscous liquid runs along its surface, forming patterns of tortured faces while tendrils of energy rage along the outline of the portal. Screams could be heard emanating from it.
The gate was opened by the combined efforts of the Prime Evils after they reunited once again. Diablo passed through it into the Pandemonium Fortress, from which he would travel into Hell. Baal departed for Mount Arreat while Mephisto stayed behind to guard the portal. He was defeated by a group of heroes, who headed through the portal in their pursuit of Diablo.
When players enter the Durance of Hate Level 3, they can already see a glimpse of the gate when they stand on the edge of the pool of blood. However, they can not enter it until Mephisto is defeated. When Mephisto is defeated, the portal will start 'coming to life' and a bridge of bones and skulls will rise from the blood-pool, creating a passage for the player.
Entering the gate starts the new act after the cinematic. The player finds him/herself in the Pandemonium Fortress. Short of using waypoints, this gate is the only way to reach Act IV from another act.