The Location
The strange relic is located in Chapter Five of "Uncharted: Drake's Fortune." The level is a massive fortress occupied by thugs trying to kill you. The level starts with some wall scaling and a quick fire fight with the above mentioned thugs. After killing the tugs, make your way to the staircase that rests against the inside of the outer wall of the fortress. Before you can make your way to the relic, you will have to fight -- and kill -- another collection of thugs that stand on the walls. Once you have killed them, position yourself so that you are looking out towards the ocean.
Moving Forward
Once you are facing the ocean, walk down the stairs located to the left of the wall. At the bottom, turn your back to the ocean. You will see a shaded structure -- small in size -- resting in front of you. A gap rests between you and this point. Get a running start and leap over the gap. "Uncharted" is quite liberal with jumping lengths, so a decent running start should be enough to clear the gap. Once you have cleared it, you will see a small treasure chest, similar to the ones that hold other relics and items of interest.
Obtaining The Relic
The treasure chest will have a "Triangle" icon next to it, which simply means you must press the triangle button on your PS3 controller to open it up. Once you open the treasure chest up you will find the strange relic resting inside. The strange relic is actually a precursor orb, similar to those found in the Jak and Daxter video games. However, this is not the end of the level -- and far from the end of the game.
Purpose
Collecting treasures -- such as the strange relic -- throughout the "Uncharted" game will earn you trophies. Trophies are a way of displaying your skill at a particular PS3 game. However, if you were to collect every single treasure within the game or none at all, it would not make a difference in regards to beating the game. In other words, you can defeat "Uncharted" while ignoring every treasure there is to find.