Darcon is a prisoner in the Bavelock prison. He plays a part in the main story of the game.

When you and Caleb must escape Bavelock when it is in lockdown, Farimir mentions someone in the prison who might know a way out of the city.

To get past a guard at the prison you first put on the city guard uniform. You find Darcon in the prison, standing alone in his cell. Caleb then opens Darcon's cell with his magic and Darcon reluctantly shows you a building with a locked door.

He then walks away saying that getting inside is your problem. Caleb opens the door and inside is a trapdoor leading down to the Thieves' Passage.