The portal is orange and energy resembling terrible silver-black viscous liquid runs along its surface, forming patterns of tortured faces while tendrils of energy rage along the outline of the portal. Its most striking feature, however, is the tortured screams of numerous humans: men, women, children alike, emanating from the portal, forming a terrible mixture of ominous sounds.
The portal's creation was witnessed by Marius after he was charged with the duty of destroying Baal's Soulstone by going to the Hellforge. It was here that he witnessed the Dark Wanderer's transformation into the physical manifestation of Diablo. This drove him insane and led to the events leading up to the expansion.
The portal was created so that Diablo, the Lord of Terror could go to the Burning Hells and rally the demonic forces for the invasion of Sanctuary, while Baal retrieved his Soulstone to capture the Worldstone which was the only thing preventing the Burning Hells from spilling into Sanctuary. Mephisto decided to watch over the portal while his brothers accomplished their missions.
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, a major plot hole considering this is the same portal that Diablo entered, and he cannot have ended up in the Fortress but in his Lair. Short of using waypoints, this gate is the only way to reach Act IV from another Act.
It is notable that during the above mentioned cinematic, Mephisto chants exactly the same words found in the Introduction section of the Diablo instruction manual.