Crucible

Crucibles are a type of energy-based melee weapon first featured in Doom (2016), discovered via hallucination in the Lazarus Labs and later obtained by the Doom Slayer, though never used as a weapon in-game. In Doom Eternal, one of the Crucibles is the Doom Slayer's own sword from his time as a Night Sentinel and can be used in the later parts of the game.

Crucible blades are said to be the only kind of weapons capable of permanently killing Titan demons; however, the Titan will rise again if the Crucible blade is ever 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.

Description
A Crucible is a 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 hellish 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 used to belong to him a long time ago. Compared to the Crucible of the previous game, it appears much less organic and monstrous, being made of simple dark grey metal instead of demonic flesh and bone. 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; it is also revealed that a Crucible is one of the few ways to permanently kill a Titan.

A Crucible can be "broken" if the handle is wrenched forcefully enough while the active blade is embedded in something. In this case, the weapon remains locked in its inactive state, requiring a two-step process to restore its functionality.


 * 1) A certain metal disc must be acquired and held near the emblem on the hilt of the Crucible, causing the former to disintegrate into red energy that is absorbed by the hilt. This makes the Crucible's emblem glow red and allows it to open into its "active" position, although it still won't be able to generate a blade. Each Crucible may require a "charger" specifically marked with its wielder's emblem for this process, as Hayden advises the Doom Slayer that "Your vault should have what you need."
 * 2) Once the hilt is reactivated, it must be dipped into a large pool of what appears to be molten metal (or possibly concentrated Argent Energy). After a few seconds, the energy blade will form over the hilt, restoring the Crucible to full functionality.

The demonic Crucible that Hayden took from the Doom Slayer was used to fuel the weapon development of the Armored Response Coalition, being the last source of Argent Energy in the Solar System. With humanity deprived of its Argent-dependent energy grid after the loss of the Well, Hayden invented a method of Argent-Synthesis, a synthetic replication that could recreate the high-yield capability of Argent Energy. Using the Crucible's hidden power, presumably both the energy absorbed when the Doom Slayer set the Elemental Wraiths' souls to rest, Hayden utilized the artifact as an Argent conductor to restoring the production of Argent Energy to Earth.

Other Crucible-like energy blades are used by the Night Sentinels, forming the points of the spears they wield as personal weapons and on their Atlan war machines; the Betrayer owns a warhammer with a head formed of similar energy. The Marauder has an axe that has some visual similarities to the demonic Crucible, including a skull head that generates a large red energy blade. It is unclear if these are the same type of weapon as the proper Crucible, however.

Doom (2016)
The Crucible first appears in hallucinations when the Doom Slayer touches the Praetor Suit and 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 set the souls of the Wraiths to rest, deactivating the energy sources powering the portal between Hell and the UAC Base on Mars. The Crucible seems to absorb energy with each Wraith set free; the gems adorning it turn from green to red, glowing brightly and producing flames, and light emits from the eye sockets of the skull.

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.

Doom Eternal
There are two Crucibles in Doom Eternal.

The one from 2016, which Hayden refers to as the "demonic Crucible," is kept in the main ARC Complex following Hell's assault on Earth; the Doom Slayer takes it with him on his way to recover Hayden's remains. After Khan Maykr cuts the power aboard the Fortress of Doom, the 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, commenting that the Khan Maykr was unaware that it was in the Slayer's possession when she set her latest plan into motion.

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 Taras Nabad, 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.

Since the Slayer once again broke off the blade of his Crucible when he defeated the Icon of Sin, the weapon is not usable in The Ancient Gods - Part 1. It is as yet unclear if he still possesses the weapon. The fate of the demonic Crucible is also unclear.

Gameplay
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. Unlike the Chainsaw, the Crucible uses one charge for each enemy, regardless of its class.

Trivia

 * The Crucible appears to be loosely designed after the central African Ikakalaka, which bears the same iconic flaring at the tip of the blade.