Thinuan

From Syra D&D Wiki

Thinuan is a dark, glittering metal with the strength and weight of steel. It is extremely rare, being found almost exclusively in the Outer Planes, but its true value comes from its unusual property to attract souls. A creature who dies while in physical contact with a piece of thinuan, or a weapon or object crafted from it, will have their soul trapped within it. The advantage to this is that raising someone from the dead is considerably easier when the caster has the thinuan, as the soul is close at hand. However, magical resurrection is impossible without it, as the soul cannot otherwise escape its thinuan prison.