To get this achievement you must first get to stage 7 and complete the teleportal event. Once done a Celestial portal will open up that you can go through. You will then have to make your way down to the bottom of the realm where you will encounter the obelisk. Interact with it and you will be rewarded with a new character (the Mercenary), 5 Lunar coins and your achievement. This can be done on the easy difficulty. Also your run will end once you do this so make sure you are sure you want to do this.
