The Forgotten City

This is story-related, you will unlock it after you talk to the person inside the Great Temple at the top of the city (I'm being vague to avoid spoilers). To enter it, you must collect all four plaques and insert them into the obelisk outside of the entrance.

Here's a spoiler-y guide on how to get all four plaques: https://gameranx.com/features/id/247675/article/the-forgotte...

For those who don't want spoilers, here's a summary on how to get them:

- Roman Plaque - behind a gate in the caves that you can access from the slum area. You either need to have the key to open the gate (there are multiple ways of getting this), or else have the Golden Bow to be able to bypass it.
- Greek Plaque - you get this from inside To do so you will have to This is a long process that I won't detail here (it essentially requires you to have completed most of the game's side quests, and then to start a new loop and immediately send him to re-do them for you). Also note that you pretty much have to grab the plaque and run for the portal ASAP to reset the loop, as the person you let loose will trigger the Golden Rule within seconds of getting out.
- Egyptian & Sumerian Plaques - you will get these by following the side quest to find Khabash. You can find him by following the hidden path inside Note that the door to the hidden path is partially camouflaged, it's directly behind the shrine (crouch down to go through it).

Once you've opened the Great Temple and are inside, approach each door in succession and choose the dialogue options that include the following at each door: 1. Pluto 2. Hades 3. Osiris 4. Nergal. Once you go through the last one and begin talking to the person in the room, you should get the achievement.