Crucible

The Crucible, a central plot item in Doom (2016) and in Doom Eternal, is an artifact from Argent D'Nur which wields mysterious control over the Argent energy reserves of Hell, capable of shutting down&mdash;and apparently reactivating&mdash;the source of that energy known as the Well. Samuel Hayden sends the Doom Slayer back into Hell with the purpose of reclaiming the Crucible, stating that its recovery is the only means to seal the Hell portal which was opened at the Argent Energy Tower by Olivia Pierce. The Slayer actually sees the Crucible several times before finding it, in bizarre psychic visions which plague him during his journey between different areas. The first such vision is seen as he dons his armor.

True form and origin
The Crucible is found at the end of the Necropolis within the Hell guards' arena, where it is stored inside a retractable pillar. Defeating all three of the Hell guards allows the Slayer to at last reclaim it, after which he encounters the spirits of five of the Night Sentinels in a green, ghostly form. It appears as a compact skull-like ornament connected to a base by a central handle. The true form of the Crucible however is not directly revealed until the end of the game, when it is taken from the Doom Slayer by Dr. Hayden forcefully while he is being held captive by the tethering device. Stating that he is unable to kill the Slayer, Hayden still manages to activate the Crucible in a threatening display, revealing it to be a weapon in the form of a sword with an Argent energy blade, inscribed with blazing red runes.

While an important item in the story, the Doom Slayer never gets to wield the demonic Crucible actively as a weapon, only briefly using it to destroy the captured souls of the Wraiths which power the Well. Hayden intends to use it to further his Argent energy research and further endanger mankind in the process.

If a stone tablet in Samuel Hayden's office is carefully examined, it can be found to display an etched picture of the Crucible in its true form. This is the only hint that is given of it before its appearance in the game's ending. The multiplayer map Argent Breach also contains a statue created by the culture of Argent D'Nur which depicts the full form of the weapon. Argenta engravings which can be found in areas such as Empyrian and Taras Nabad frequently depict the Crucible against the backdrop of a diagram which resembles the mystic as it appears in  tradition. As statuary strongly associated with the Maykrs sometimes depicts the weapon, it may have some historical connection to them.

The Slayer's mark somewhat resembles the Crucible.

Doom (2016)
As stated previously, the Crucible serves as the main MacGuffin in the game and is seen primarily in visions, out of reach of the Slayer. The Slayer finally acquires the Crucible after facing off against the three Hell guards encountered at the end of the Necropolis. He retains it from that point into the end of the game, using it like a key to shut down the power source of the Hell portal in Argent D'Nur. After the defeat of the Spider Mastermind, Samuel Hayden abducts the Slayer and confiscates the Crucible from him. It is at this point where the true form of the artifact is revealed.

Doom Eternal
In Doom Eternal, the Doom Slayer recovers the Crucible from the ARC Complex, where it had been left by Samuel Hayden for the Slayer to retrieve. He then uses its ability to produce an infinite supply of Argent energy as a power source to restore the Fortress of Doom to operational status after the Khan Maykr drains it of all its Sentinel energy. This causes Hayden to comment that the Khan did not know the Slayer had "the demonic Crucible", but also that it is now needed to retrieve the Slayer's own personal Crucible from Taras Nabad where it was left in a Titan's body.

The Slayer's Crucible
The Doom Slayer's Crucible has a dramatically different hilt, with a Sentinel design and bearing the mark of the Slayer. The energy blade is nearly identical to that of the demonic Crucible, save for a small hole at the bottom near its attachment to the hilt.

Other
Marauders can also be seen to carry a similar Argent-bladed battle axe, and Night Sentinel guards are equipped with Argent-tipped spears. The Betrayer is shown to have what appears to be a crucible hammer, but it is only seen laying off to the side, never in use.

Pre-release
According to The Art of DOOM, the Crucible was referred to as a "talisman" during development.