For this achievement, you actually require several others and should be an Archer for the first part of this.
You are given the Eternal Wakestone item by killing the Sphinx NPC, who can be found up in the mountains (Guide for her questline by the excellent Powerpyx:
https://www.powerpyx.com/dragons-dogma-2-all-sphinx-riddles-... ) whereupon she tasks you with completing ten of her riddles. After the fifth riddle, she flies off, and you should grab onto her and hold on for the journey while refilling your stamina with food, landing at the next location for her final 5 riddles.
(I've also read you can destroy the jewellery on her body without her flying off to kill her, but as the game is new, try this at your own risk.)
After you finish her 10th riddle, she bids you farewell. Let her fly into the air while aiming above where she'll go. Using the Unmaking Arrow from one of her reward chests, equip it in the inventory and shoot her with it and she will die and the big chest will unlock, granting you this item. I recommend making an inn save just before you turn in the final riddle, so you can reload if you mess up.
Now, with the eternal wakestone, go to the morgue in a city or settlement where you've killed some NPCs. For me, I had killed two in the Capital, so I ventured there. Now, while standing close to the morgue make sure you're OUTSIDE (your Arisen throws the stone into the air), and use the eternal wakestone from your inventory. The game will prompt you about who you're reviving, accept, and you'll resurrect them and the achievement will pop. As far as I know, you need to do this with more than one body in the morgue, as it didn't work for me with just the one body and I had to reload.
Note: It's not a glitch, you won't unlock THOUGHT I'D LOST YOU doing this. That achievement requires you to revive a single individual NPC at a morgue using a standard Wakestone.