So you might be wondering why you are not getting that challenge chest and what you need to do during an encounter to get that extra chest, well look no further!
Abe: Kill the 4 phases before he hides the Challenge chest (Timed event, invisible Timer), does not matter if you die.
Dragon Rescue: The Dragon must survive, does not matter if you die.
Rak'var: Must survive + not allowed to run to far away from the area Rak'var spawns in.
Dragon Tower: Must Survive, not allowed to die during the encounter.
Troll King: Must kill the doggy he spawns(Jackyl) + Jal'kor's Retainer after he heals back to full HP before you kill him, must also open the challenge chest before you kill the Troll King, does not matter if you die (Challenge Chest becomes available after you kill the Doggy and the retainer mob, but will disapear if you kill the Troll King before you open it)
Bleld: Must survive, if you die the Challenge Chest is gone.
Nazich: Must survive, and must kill him before he summons a 5th minion.
Syerl'Khan: Kill at least 15 Nether Casters, 4 spawns each time new tentacles spawns, does not matter if you die.
If you complete the challenge, you get to open the Challenge Chest, if you fail, the challenge chest simply disappears/wont be available.