All Black

History
The first symbiote, All-Black was manifested from the shadow of the evil deity Knull and tempered using the divine power of a slain Celestial. Taking the form of a sword made from living darkness, All-Black was wielded by Knull during the dark god's deicidal rampage, earning it the monikers of the Necrosword and Godslayer. After Knull was seemingly killed in battle against a group of golden-armored gods, All-Black was stolen by an alien called Gorr, who unwittingly followed in Knull's footsteps, using it to slay gods across the cosmos and earning the title of the God Butcher. Feeding off the slain gods' blood, All-Black corrupted Gorr into a kind of evil god himself, bestowing him with immortality and a host of other abilities. In his final battle against Gorr alongside his past and future selves, Thor was able to absorb the Godbomb's blast, bonding to All-Black and using it to finally defeat Gorr. King Thor used the last of the Thor-Force to purge All-Black from his younger self's body, and to keep Young Thor from claiming it out of arrogance cast the planetoid into a black hole.

When Galactus returned to Earth to finally devour it, King Thor used All-Black the Necrosword to defeat him. After their battle, King Thor gave Galactus leave to consume Mars; but while Galactus was devouring it, the black blade that he (and presumably King Thor and his granddaughters as well) believed had been destroyed revealed it had transferred from Thor to the Devourer of Worlds, turning him into the Butcher of Worlds.

During the Butcher of Worlds' battle against Ego, All-Black the Necrosword transferred to the Living Planet, transforming him into the Necroworld. Soon afterwards, Ego proceeded to devour Galactus, finally making him pay for all the planets he had destroyed.

While bonded to Ego, All-Black carried out its creator's deicidal mission by killing the last of the Celestials, compelling its host to travel what was left of the cosmos and devour whatever he came across. However, soon after rendering the Acanti extinct, Ego was challenged for possession of All-Black by King Loki, who would successfully claim and bond to it; going on to fight King Thor, as witnessed in prescient visions had by Ulysses and the Goddesses of Thunder.

Personality
All-Black is sentient and possesses a malevolent will of its own, though it was subservient to its creator Knull. Like other symbiotes, it is capable of communicating with its hosts and seeking to corrupt them; being indicated to have influenced the already misotheistic Gorr into becoming the deicidal God-Butcher, and corrupting Old Galactus into the monstrous Butcher of Worlds.

Powers
These are the abilities the wielders of this cursed blade have displayed:
 * Umbrakinesis: All-Black's host has the power to summon, create, control and/or manipulate darkness - presumably derived from its creator's ability to manipulate the living abyss.
 * Darkness Constructs: Similar to other symbiotes' shapeshifting abilities, All-Black can create wings, weapons, armor, tendrils, and a cloak of darkness - though its primary form is a sword. It can also be used to create semi/fully sentient entities as minions, such as the shadowy beasts known as the "Black Berserkers".
 * Superhuman Strength: Its wielder has strength that is at least comparable to Thor, if not greater, as Knull was able to decapitate a Celestial with a single strike; and Gorr was able to best young Thor, present-day Thor, and Skyfather-Thor all at the same time, and has single-handedly defeated many millions of gods in open combat, including one that supposedly "wrestled with black holes for fun."
 * Superhuman Durability: Gorr was extremely durable against all forms of trauma as he was able to survive a massive lightning bolt summoned by Thor, and the Godblast of an Odin-Force powered Skyfather Thor which propelled him "several light years" away.
 * Superhuman Speed: Its wielder can fly at blinding speeds, as when Gorr quickly overtook the thunder god on their first encounter.
 * Superhuman Reflexes: Gorr was able to catch Thor rather easily off guard, as well as many other gods.
 * Longevity: After bonding to All-Black, Gorr lived for at least two-thousand years and still looked to be in his prime.
 * Regeneration: Gorr was able to recover from a lightning strike within four days with no signs of injury from the incident. He was also able to regenerate or reattach his arm after it was severed by Thor.
 * Energy Manipulation: Shown briefly during its use, the necrosword is capable of discharging or enhancing the energy projection capacity of its wearer. This is seen when Gorr used it to blow up a Space Sharks in his battle with young Thor, while an elder Thor used it to strengthen both his Thorforce and mystical lightning when battling old Galactus.
 * Power Source / God Blood: The sword draws its strength from the blood of gods it kills. The more gods killed by it or its creations, the more power the blade can provide its user. This power must be replenished as it is used up.
 * Divine Slaying: The weapon's main purpose out of its myriad of abilities is the killing of gods by piercing their immortality or resistance to the life imperilment that comes with mortality.

Weaknesses
Knull notes that All-Black's forging gave it and most subsequent symbiotes a psychosomatic fear of intense heat and sonic vibrations.