Gram
Gram, also called the Sword of Truth, is a sword originally crafted by Regin the Smith to kill Loki.
Fafnir had become a dragon due to prolonged exposure to Andvari's Gold, and his blood also carried Andvari's curse. When the sword was drenched in his blood, the sword too absorbed the curse, revealing the truth of the heart of anyone cut with it.
The events of the sword's creation were engineered by Loki (via Time Stone powered time travel) in order to be used in the final battle against Thanos and change the timeline to prevent Verity Willis from dying.