A Crucible is an energy blade weapon powered by raw Wraith or Hell energy, or possibly Argent Energy. The handle is capable of being charged with energy, and can form a red glowing blade covered in runes that may be wielded like a sword. The Doom Slayer obtains a demonic variation of such a weapon in Doom (2016), used to presumably draw energy out of the elemental wraiths on Argent D’Nur. It is never used as a weapon, but Samuel Hayden activates the blade once he obtains it. In Doom Eternal it is revealed that more than one Crucible exists, and the Doom Slayer re-obtains one that which used to belong to him a long time ago.
It is revealed that a Crucible can be used to permanently kill a Titan. The crucible works by generating a hot, presumably super-sharp blade of energy that can cut through demons very easily, seemingly burning through them. This allows the Doom Slayer to instantly slice any demon in pieces, even exceptionally large and durable ones like the Tyrant and Baron of Hell.