The Quest portal is the ^ trap that warps you between the main Dungeons of Doom and the Quest. On a certain level of the dungeon below the branch to Sokoban, you will receive a request for help from your Quest leader. You must search for it; you could walk on every square of every room, or you might see a monster (even your own pet) stumble into it.
In some games, the Quest portal (and the leader's call for help) will actually be upon the Rogue level.
For those playing SLASH'EM, there are three additional quest portals; these go to the Lawful Quest, Neutral Quest and Chaotic Quest. There is no special message for these three portals. You must search for them, but know that no two of the four portals can be in the same level. Search from dungeon levels 15 to 20.