The Temple - Ancient Plates | Side quests Lords of the Fallen Guide

You receive the quest, and the reward, from Yetka.

After you walk into the portal to the Temple at the first gate (that is initially closed), you encounter Yetka [M3.2]. She asks you to find Ancient Tablets scattered all around the Temple. You need to find five of them and take them back to Yetka. You find her after the battle with the Infiltrator (until then, you should have collected all of them) or later in the same location, after you return from the Citadel. You will be rewarded with 1500 EXP and Shards: resistance to poison, resistance to magic, resistance lightning, of energy and of magic.

You can find the whereabouts of the plates in the chapter entitled Collectibles and items->Ancient Tablets.