The Crucible is a large dagger-like sword with a handle that looks like skull and lower part modeled after Mancubus eye. It has an ability to absorb huge amounts of demonic energy.
The Crucible first appears via hallucination in the beginning of the game when the Doom Slayer touches the Praetor Suit, and again after the player touches Helix Stone where an altar with a strange artifact and a source of demonic energy can be seen. Physically it appears at the end of level The Necropolis guarded by the Hell Guards, which must be defeated in order to reach the artifact. After this moment The Crucible appears at the end of Argent D'Nur, where player uses it to close 3 sources of energy powering the portal between Hell and UAC Base on Mars. Samuel Hayden appears to know far more about The Crucible than he lets on; when he takes it from the Doom Slayer at the end of the game he knows how to 'activate' it, causing a large plasma-red and sword-like blade bearing demonic runes to manifest from within the artifact. It is the Doom Slayer's sword.