There are 6 relics distributed throughout the game. For this achievement you only need to collect 1. However, you may as well go for all of them.
I will list the approximate position in a level and then provide videos of the exact steps needed to take to acquire the relics.
Relic #1
Mediterranean Sea: the top of the Kraken room when coming back after attaining Thor's Glove.
Relic #2
Coastal Thailand: near the end of the level when you have to move the giant column with the tree on top and the bridge attached.
Relic #3
Croft Manor: In the sewer area of the dungeons. You must use the pressure plates to open the grate to the sewer.
Relic #4
Southern Mexico: It is located in the statue of Thor's right eye. You must have already retrieved the belt and moved the hammer back to its original position.
Relic #5
Jan Mayen Island: In the maze. Go ahead and clear the maze of all thralls and go to the end. You will need to grab one of the levers and ride it down. Jump on the bike and navigate the maze before the door closes.
Relic #6
Arctic Sea: In the room before the Natla fight (the far end of the room has a red glow on the walls and ceiling) there are 4 runes. You must touch all of the runes to get the waterfall in the center to stop. You can then swing to the last relic.
Achievments Unlocked!
***NOTE: All 3 relics must be found in the same playthrough of the game to get the Dedicated Relic Collector. The same is true for Master Relic Collector in which all 6 must be found in the same playthrough.