You need to finish Darksiders 3 with 100%? At that point you need to open all trophies, accomplishments and collectibles!
In this guide, we clarify the accompanying accomplishment/trophy:
Find the Mysterious Stone Sigil
To unlock the trophy you need the mysterious relic. But you have to make a decision to kill or spare Abraxis. For this achievement you have to kill him!
Kill Abraxis and bring his soul (form of a stone) to The Lord of the Hollows. There you have to make another decision, do you believe in The Lord of the Hollows or do you want to kill him? You must spare him, because only then you will receive the mysterious relic and unlock the trophy.
If you make another decision, you will miss this trophy / achievement!