When you are in the last level Artic sea in the area where you just come out of the water at the ledge you will see huge thralls like Yeti Thralls and Viking Thralls. You can destroy them easily with Thor's Hammer. Once have delt with all of them there will be four switches once you have turned all of them on the middle water fall will turn off but only for a limited time so you can swing from the ring and jump on the ledge before it turns off and collect your Relic! When you collect it will give Lara's health will build up but not just that you will also unlock more concept art! So start searching for more! Lastly it will grant you the Achievement called Relic Collector(30)pts. ! So raid on!