On this page of our trophy guide to The Last of Us 2 you can find a detailed description of unlocking the Relic of the Sages trophy, which is earned for finding a strange relic. In our description we explain in which stage you can unlock this trophy and where the unique treasure was hidden.
The strange relic is a collectible - you can find it along with other secrets in the Hostile Territory stage. For the record, this is one of the Abby's levels which is part of the Seattle Day 1 chapter (Abby).
During this stage, reach the street shown in the picture. On the left and right, there are shops which you can explore.
Check out the last store on the left - Ruby Dragon. There's a staircase leading upstairs at the back.
You have to crawl upstairs and defeat two clickers. Then go to the large balcony. At the spot shown in the picture you have to make a jump (while running) and reach the building on the opposite side of the alleyway.
Turn left and start exploring the adjacent rooms. The strange artifact is in a poorly lit place shown in the picture. It will be an egg lying on the table.