“Defeat the Taken keeping the prison doors sealed.”
“Defeat the Taken” is an objective in the Strike The Sunless Cell in Destiny: The Taken King
Head through the now open door to move forward. Get ready for the Taken in the next room. Expect to encounter Taken Acolytes and Knights in the first wave. In the second, expect those and Taken Psions, Phalanx.
The second wave has Tortured Acolytes and a Tortured Wizard. Take out the Wizard before it can summon too many Shadow Thralls. Quickly deal with them and the Acolytes after you kill the Wizard. On the left-hand side of the room you will have to deal with a Taken Portal. Get to that after the fireteam has dealt with the Wizard.
The final wave will bring in Urlot, the Trampler. This Taken Knight is tough. Go for headshots and be wary of his boomer and Fiery Gaze. He is nothing special beyond a tougher Taken Knight. He can easily be stunlocked and taken out like that.