In Outland Edit
TBC This section concerns content exclusive to The Burning Crusade.
The Twilight Portal is one of the four dimensional gateways which Magtheridon used to bring demons from the Twisting Nether under his control. It is located on Twilight Ridge and is involved in the quest N  Disrupting the Twilight Portal in which you must attack Deathshadow agents. It is still intact but not functioning, so the Deathshadow are channeling magic into it in order to stabilize it.
However, for the quest N  The Soul Cannon of Reth'hedron, upon defeating Reth'hedron the Subduer, he opens the Twilight Portal and quickly escapes through it vowing to one day return.
In Northrend Edit
WotLK This section concerns content exclusive to Wrath of the Lich King.
Twilight Portals are also summoned by Tenebron, Shadron, and Vesperon in The Obsidian Sanctum, allowing players to enter the Twilight Realm.