The Dark Portal remained dormant for twenty years, until an artifact of incredible power was found by the demon Lord Kazzak, a lieutenant of Kil'jaeden. Kazzak used the powerful artifact to reopen the Dark Portal to Draenor and cross into Outland.

Other Dark Portals
A few whisper that the Dark Portal that first brought the demons to Azeroth (during the War of the Ancients) yet remains and rests under the sea waiting for someone to reopen it.WRPG 177

Kael'thas and Lady Vashj escaped with the blood wow gold elves from Dalaran through the Portal to Outland during The Crossing.

The Portal in World of Warcraft
When World of Warcraft launched, the Dark Portal was a noticeable (but unusable) feature in the Blasted Lands, located southeast of Stormwind in the Eastern Kingdoms. The Portal is located in the southeastern corner of the Blasted Lands and (until Patch 2.0.1) was surrounded by elite demons and servants of the named demons, and the glow around the Portal's gateway was blue, indicating its inactivity. It was considered to be decorative only, a piece of Warcraft lore to look at. The Portal on the Azeroth side appears to be rather simple - the only decoration on it being the two "guardians" on either side of the gateway and the coiled snake-head above it.