Inside the temple cellar, there is 3 doors, with 2 has 1 pedestal each in front of the door. The door without the pedestal in front of it can be open directly. Once you open and get inside, you see a pedestal with a teleport device. That will port you to a room of mirrors for your trial as a dragon initiate.
And regarding your level, you are lvl 20 not before even facing amdusias? I guess you might overdone it by killing too much over at fjords. You will get peanuts exp with sentinel island mobs like this.
