To get the trophy that's called "Memories of Eregion", you will need to find, then climb to the top middle of the tower and activate what's called a Forge Tower.
IF you have played an Assassin's Creed game in the past, then this job should be very familiar. All of these Forge towers can be found in both maps (Udon, Num). If you have a hard time finding them, just use Wraith Vision, which can be activated by pressing
.
Once you find a Forge tower, rush towards it and then start climbing until you've reached the middle of the tower (not all the way up to, but right before it). Then it's just a matter of using the Reforge technique that's prompted when you reach the object. To Reforge, simply hit
and Celebrindor will strike the object with a hammer and it will be forged for good.
If you need help finding locations on every one of the Forge locations, the best way to serve you is with a visual guide. I found a suitable video that will aid you in this regard. All credit goes to VGFAQ for the video.
Happy hunting and good luck!