You should have obtained the Deserted Temple questline from Defense Captain Auston in Bone Reef Peninsula.
Then, head over to Deserted Temple and talk to the NPC in there. Once that's done, you will need to kill some mobs and the first boss before submitting the quest to another NPC in DT.
You will then get another quest to kill more mobs and the 2nd boss before running back to the previous NPC.
Then, kill the last boss in DT for the quest and head over to Jale and find Stan to submit the quest, before running back to DT and run towards Cassidy's chamber.