MISSABLE: Once in Outlands theres a cave near to where you start called Desert Cave. Go inside and talk to Selenn and get his side quest which has 4 parts. The one you need for this trophy is called The Reaper, simple activate this as the active quest and find your way to the quest marker.
Once inside make a manual save.
Candle puzzle on floor. Light the candles so they match what is lit under the statue to the far left. Open chest and get a tough Demon fight.
Make another save after beating it. Next comes the boss fight you have a choice to make to kill or not to kill. At this point not sure if choice matters for trophy but i choose to kill. But make a save after candle puzzle to be safe.