Crucible

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 of the Doom Slayer is partly destroyed when the Doom Slayer breaks the blade to finish off the Icon of Sin, leaving only the handle intact.

The crucible blades are supposedly the only kind of weapons capable of permanently killing Titan demons.

Description
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) and 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 exist, 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 one as a hammer.

Doom (2016)
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 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 confiscate 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.

Doom Eternal
There are two Crucibles in Doom Eternal. The one from 2016 powers the Fortress after Khan Maykr cuts the power source and unleash demons upon the Slayer, the other which originally belongs to Doom Slayer can be used as a weapon in Doom Eternal. The Crucible contains three "charges" located just below the crosshair, The Crucible acts much like the chainsaw in its ability to instantly kill any Demon other than the marauder for the cost of one of the charges, and a red sword pick-up is used to refill a single charge.