Yes, Darklightr, in the Secret Passage ~V~ is indestructible--you cannot kill him so don't bother with him. You must kill everything else (with an exception, depending upon your toon & skills). Kill everything outside of the cells first, then kill everything in the cells. You're best-served if you have a toon that can attack from outside the cells without opening them. Ranged spells or archery skills from the Hunting mastery (Puncture Shot goes through the cells bars/doors) allow you to stay out of harm's way while annihilating the monsters. The exception is Toxeus, Arthur's second form. If you decide to kill Toxeus, you must be careful in regards to his reflection aura. He will reflect damage back to you, which can kill you if you're not careful. Some people will kill everything except for Arthur, then drop a Monster Lure just as Arthur's first form is about to expire. This (sometimes) causes Toxeus's form to focus on the ML, allowing you to raid the treasure chest. If the ML is dropped near Toxeus's cell, you'll be out of his aggro range when you're raiding the chest. However, in my experience, Toxeus doesn't always want to go after the ML, so it's a bit hit-and-miss with that tactic, although it's a way to grab the loot without having to kill Toxeus, who is simply a very cheeky little monkey, particularly in higher difficulties of the SP (and obviously, you must open the cell door to use the ML tactic).
So, if the chest didn't open for you, you obviously didn't kill everything (~V~ not included as he won't die).