Crucible

The Crucible, a central plot item in Doom (2016), 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 marine 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 Doom marine actually sees the Crucible several times before finding it, in bizarre psychic visions which plague him during his journies between different areas.

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 Doom marine 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 marine by Dr. Hayden forcefully while he is being held captive by the tethering device. Stating that he is unable to kill the marine, 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 marine never gets to wield the Crucible actively as a weapon, only briefly using it to destroy the captured souls of the Wraiths which power the Well. It is currently presumed that Doom Eternal may involve the marine attempting to reclaim it from Hayden, who 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.

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 Marine. The Marine 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 Marine and confiscates the Crucible from him. It is at this point where the true form of the artifact is revealed.

Related artifacts
In Doom Eternal, the Doom Slayer possesses a similar but distinct sword (likely known as the Unmaykr). While its Argent energy blade is nearly identical, save for a small hole at the bottom near its attachment to the hilt, the hilt itself is dramatically different and bears the Slayer's mark. It briefly appears in the E3 2019 story trailer, as it is going to be used against the tyrant and is seen in full effect during the 2020 story gameplay trailer against another tyrant.

The Marauder can also be seen to carry a similar Argent-bladed battle axe, and several Night Sentinels also seem to carry Argent-tipped spears. The relationship of these weapons to the Crucible is currently unknown.

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