The Crucibles are a form of energy blade weapons featured in Doom (2016), discovered via hallucination in the Lazarus Labs and later obtained, though never used as a weapon in-game. In Doom Eternal, one of the Crucibles is the Doom Slayer's sword and can be used in the later parts of the game as an immensely powerful weapon.
The Crucible blades are supposedly the only kind of weapons capable of permanently killing Titan demons; however, the Titan will rise again if the Crucible blade is removed from its body. To circumvent this, the Doom Slayer tends to break the handle off of a Crucible after killing a Titan, leaving the blade embedded. When this is done, blue energy visibly spreads over the Titan's body, seemingly to indicate its permanent neutralization.
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. Crucible blades are also used by the Marauder in the form of an axe, and lances and spears with Crucible blades are seen used by the Night Sentinels. Valen also uses similar technology for the head of his warhammer.
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 the 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 The Necropolis, embedded in a pillar protected by the Hell Guards. After recovering the Crucible, the Doom Slayer uses it at the end of Argent D'Nur, to deactivate the Wraith-derived energy sources powering the portal between Hell and the UAC Base on Mars.
Samuel Hayden appears to know far more about the Crucible than he lets on; when he confiscates it from the Doom Slayer at the end of the game he knows how to 'activate' it, causing a large red energy blade bearing demonic runes to manifest from within the artifact.
There are two Crucibles in Doom Eternal:
The one from 2016, referred to by Hayden as the "demonic Crucible," is kept in the ARC base following the Hell assault on Earth; the Doom Slayer takes it with him along with Hayden's remains. After Khan Maykr cuts the power aboard the Fortress of Doom, the Doom Slayer uses the demonic Crucible as an alternative power source, allowing him to restore his base's functionality (albeit with red lights instead of blue). Hayden notes the irony of the Crucible (a powerful source of Argent Energy, which the Doom Slayer has vowed to destroy) being used to facilitate the Slayer's mission.
The second Crucible is the personal weapon of the Doom Slayer himself, having been wielded during his time as a Night Sentinel. He lost the weapon after using it to slay a Titan on Argent D'Nur, which marked the start of his legend among the Sentinels. Needing the Crucible's power once more, the Slayer returns to the scene of the battle and breaks off the hilt of his weapon to avoid resurrecting the Titan. After reforging the Crucible's blade and marking it with his emblem, he can use it as a weapon, notably wielding it to deliver the final blow against the Icon of Sin.
In Doom Eternal, the Crucible acts much like the Chainsaw in its ability to instantly kill any Demon (other than a shielded Marauder) for the cost of a charge. It can store up to three charges at a time, which are displayed in a meter just below the Doom Slayer's crosshair. Red sword pick-ups are used to recharge the Crucible, with each pickup equalling a single charge. Compared to the Chainsaw, the Crucible is effective against a wider range of enemies, but lacks the ability to drop pickups on kill.