Soul Cube/Doom 3

"Destroying evil gives us life and makes us stronger. Unleash us when you hear our call. We will pass the life force of those we slay to you. We foresee a great battle. The outcome is not clear. We are the only way to destroy Hell's mightiest warrior."

- The Soul Cube

The Soul Cube is the most powerful weapon in Doom 3. It is an ancient Martian artifact, taking the appearance of a levitating, sentient cube-like object with ornate decorations, that produces spiraling metallic blades when it's in operation. The player first obtains this weapon in Hell after defeating the Guardian boss.

Background
"An artifact discovered in the caverns excavation site. Research of the Cube's origin and function are still ongoing."

- PDA Description

After the Martian race first encountered the forces of Hell at some point in their past, they struggled in a hopeless battle against its demons. With whatever conventional weapons they had proving ineffective at stemming the horde, they eventually devised a plan using their greatest innovation, which bordered on functional magic. Most of the survivors were sacrificed, and their souls were used to create the weapon that would turn the tide: the Soul Cube. With it, their single greatest champion, simply called "the Hero," (a take on the Doomguy's namelessness) took on the invading force singlehandedly and managed to seal the portal. "The Hero" didn't seem to survive, since his burial chamber was found. He appears to be very tall, because his tomb is very long. The Cube was then buried along with other artifacts near The Hero's burial chamber, in case anyone ever had to deal with the forces of Hell again.

Ages later, human archeologists on Mars discovered the Soul Cube and, through the translation of the Martian tablets, learned of its purpose. Director Larry Bullman was the one who first theorized that the Soul Cube was an ancient weapon.

During the game
Shortly before the Mars invasion began, Dr. Betruger took the Soul Cube and entered the teleporter, without authorization, to have it contained in Hell. Now in possession of the only weapon that could defeat them, the demons invaded the UAC Mars Base.

By the time the Marine arrive in the Delta Labs, he meets a research specialist, Ian McCormick, who reveals that the only way to subdue the invasion is to go back to Hell and retrieve the Soul Cube. The Marine was eventually successful in retrieving the Soul Cube at the end of his trek in Hell. After the player obtains the Soul Cube, it gives a speech, with the most important words informing the user that they (the souls of the Cube) are the "Praeleanthor" (possibly the name of the Ancient Martian race) and that they are the only way to defeat "Hell's Mightiest Warrior" (the Cyberdemon).

"We are many. We are one. We are the Praeleanthor. You know us as the Soul Cube. Free us from our eternal prison and we will help you. Vanquish our enemies and we grow stronger. Listen for our call and then free us, to smite down the evil."

With the Cube's aid, the Marine was able to seal the main portal to Hell in the Delta Labs. But not long after, he realizes that Betruger opened yet another Hell Hole in the Caverns. Eventually, a UAC archaeologist named Pierce Rogers realizes the player has the Soul Cube. Knowing that the Cube can only destroy the only active Hell Hole, he grants the player the access to enter the Caverns to confront the Cyberdemon that guards the portal. After a great battle with the Cyberdemon and its minions, the Marine and the Soul Cube triumph.

The Cyberdemon is slain and the Soul Cube proceeds to travel into the dimensional portal to seal it off, causing lava to boil up to the top and solidify. The Cube's ultimate fate is not clear. It was either destroyed (sacrificing itself, and perhaps also freeing the souls imprisoned in it) or trapped within the portal in order to destroy Hell's only link to the human dimension.

Weapon schematics
The Soul Cube has the ability to sense death. It can absorb the lifeforces of any fallen entity, demon or human, to make itself operational. The five orbs located on the sides indicate the amount of lifeforce it has absorbed.

Once all five orbs are lit, the Soul Cube is operational, which the Cube will respond with "Use us!" Through a telepathic link with the user, the Cube then displays a large intersecting collection of whirling circular blades before zooming off to the target, passing through obstacles and damaging it with the said blade array. This process will cause the Cube to steal the target's life force to transfer it to the user, partially or completely healing any injuries the player may have, depending on the remaining hit points of the main target, at a maximum amount of 1,000 (or splash damage of 40 to adjacent enemies). The maximum health that the Cube can restore to the player is 99. The rate of restoration is 5 health points per interval, and two intervals per second. The Soul Cube will not restore damage taken after the attack begins, but before it is completed.

The Soul Cube does not store extra lifeforces when it's fully charged. After five lifeforces are absorbed, the Cube will not absorb the sixth lifeforce until it is activated by the user.

Properties

 * It is immune to radiological scans of any nature, including X-rays and gamma rays.
 * Its atomic makeup cannot be analyzed for this reason.
 * It maintains a constant temperature of 98.8 degrees Fahrenheit (37.1 degrees Celsius), regardless of surroundings (as the natural, constant body temperature of a human being is 98.6 degrees Fahrenheit, then 98.8 degrees Fahrenheit is likely the natural, constant body temperature of a Martian being, since the cube is composed of Martian souls).
 * It levitates (at least it does after the player acquires it; one scientist described it as "an elaborate paperweight")
 * This would make it rather difficult to measure the Cube's mass. Theoretically, the Cube could be launched at a constant speed to measure impact force, from which its mass could be inferred. Since "all attempts to determine its mass have met with no success," it was probably tried, among various other methods.
 * It is apparently immune to damage, at least from conventional sources.
 * It is not a perfect cube. Portions of it are ornamentally decorated, and one entire corner of the Cube has been cut into with ornamental sculpting that seems almost organic, resembling a face with four eyes.
 * It possesses very high armor penetration, being the only weapon capable of damaging the otherwise invulnerable Cyberdemon.
 * Despite very sophisticated attempts to open and scan the Soul Cube, nothing was discovered about the Soul Cube's operating mechanisms or interior until the player found it.
 * The Cube does not store "extra" energy from kills beyond the required five (in other words, killing ten enemies does not allow you to use it twice in a row).
 * It does not actually use the souls of its user's slain victims to power itself. Rather, as Larry Bullman explains in his report, the Cube "senses" deaths at the hands of its user and somehow extracts energy from those events.
 * The Cube does not have a physical firing mechanism, but it follows the wishes of its owner precisely. This may indicate that it shares some kind of psychic connection with its user.

Tactical usage
The Soul Cube should be reserved for larger enemies, like the Hell Knight or Arch-vile, and bosses like Sabaoth. Since the Soul Cube requires five kills before it becomes operational, it is advisable to use it primarily on stronger enemies depending on the type of monster you are having trouble with.

When charging the Soul Cube, the best enemies to kill are ones with few Hit Points, such as the Trite, Tick, or Lost Souls, although it is also possible to fuel up the Soul Cube by killing humans.

Only the boss demons, like the Vagary, Sabaoth and the Cyberdemon, can hope to survive even a single Soul Cube attack (4 attacks to be exact to kill the Cyberdemon), and even then the Cube still performs a life force transfusion. The Cube then retracts the array and returns to the user until its wielder kills enough enemies to re-energize it.

It's worth noting that the Soul Cube will only work against enemies that have been alerted of the player's presence. If an enemy is still unaware of the player and the Soul Cube is thrown at it, the artifact will only fly in that direction and return to the player, doing absolutely no damage and wasting that charge. A single shot from any weapon, making noise while running around, or simply turning on the flashlight can make enemies to become aware of the player and thus allow the Soul Cube to be used against them.

Trivia

 * The Soul Cube can be seen on a data file displayed on a computer screen in a locked room during the first level of Doom 3.
 * The concept of feeding a weapon with the "souls" of killed enemies might be inspired by the Doom Bible. At the ammo section, it was told that the demon-styled weapons (like the former Unmaker) used the "Killed humans" ammo.
 * The Soul Cube appears as an Easter Egg in DOOM and Doom Eternal, in the level Lazarus Labs and Fortress of Doom.
 * On Nightmare difficulty in Doom 3 and in the The Lost Mission expansion, the Soul Cube is given to the player at the beginning of the game to compensate for the constantly depleting health.
 * In the same difficulty, the Soul Cube will charge if Sentry Bots become disabled in combat. This is apparently odd due to the Sentry Bot's inorganic nature.