Before Kingdom Hearts Union χ
Long ago, Darkness possessed physical forms, but cast off these forms to defeat Light and became shadowy incorporeal entities. In order to propagate and increase their numbers, they attacked the hearts directly, with people succumbing to their negative emotions and giving rise to the Pureblood Heartless. There were initially thirteen main Darknesses, but the Keyblade War reduced this number to six, with the strongest seven locked away in the hearts of the Foretellers, Luxu, and the Master of the Masters. At some point later in time, when the Master of Masters was composing the Book of Prophecies utilizing knowledge passed to him via the Gazing Eye, Darkness watched over his shoulder, reading the passages as they were entered. This is how Darkness came to learn that, in the future, Ventus would harbor Darkness. Additionally, the two had spoken to one another in the past, though because of their opposing positions they were unable to understand each another. Even so, Darkness views the Master of Masters as an "old friend".
Kingdom Hearts Union χ
Before the Dandelions could cross worldlines in order to escape the Keyblade War, Darkness took Ventus as its vessel, helping him obtain his unconscious desire for power by striking down one of the new Union leaders, Strelitzia, and stealing her Rulebook. Darkness then took on the form of Ava and delivered the Rulebook to Ventus near the clock tower, instructing him to meet the other new Union leaders at the Keyblade Graveyard at the allotted time.
After reliving the events of the past for two years, the old world's timeline concerning the Keyblade War was eventually rewritten. Going forward, Darkness approaches Maleficent after she is defeated by the likes of Player and Prince Phillip. Darkness explains that the reason she could not rewrite history was because the Master of Masters had known she would arrive from the future in order to try and alter the past, which is why Enchanted Dominion had been excluded from the real world until after the Keyblade War. As Darkness feared Maleficent would return to the future with knowledge of the past, it trapped her within the data of Enchanted Dominion and ensured that the data of Radiant Garden was excluded.
Darkness then guides Maleficent to Data Daybreak Town, where she is to use the lifeboat hidden within the clock tower, which had been intended to be used to cross worldlines - though it mentions that the lifeboat had never been finished, thus allowing her to escape to a different time rather than a different world. As she would lose her body as a result of the incomplete nature of the lifeboat, Maleficent would break through the time barrier as she travelled at the speed of light, requiring only someone with memories of her as a medium to recreate a body in the future - which is guaranteed as the Book of Prophecies dictates her existence in the future. When Maleficent questions Darkness's identity, it calls itself an "old friend", explaining that it needed her back in her own time to act as a waypoint, thus allowing someone to travel through time into the future.
Upon locating the lifeboat, Maleficent hesitates to return to her own time, thus allowing Lauriam to catch up with herself and Darkness, wherein she reveals the function of the lifeboat as a means of escape to Lauriam before attempting to leave. However, Lauriam doesn't intend on allowing this to happen, and he attacks Maleficent, who is aided by Darkness in defeating the Keyblade wielder in order to utilize the lifeboat to return to the real world. Once there, Darkness instructs Maleficent to utilize the real lifeboat to get back to her own time, but refuses to join her, as it has something it still needs to do in the past.
Returning to Data Daybreak Town, Darkness reveals itself to the Union leaders after Ventus collapses, fuelled by Lauriam's return and the guilt of Strelitzia's death. When questioned as to its intentions, Darkness explains that it does not have anything that it is trying to achieve, to which Lauriam asks why it killed his sister. Elaborating, Darkness reveals that it had been an unconscious desire of Ventus's to obtain power, and that they had simply done what was necessary in order to give him that, meaning that there was no real reason as to why Strelitzia had to die. Enraged, Lauriam attacks Darkness, but he's defeated easily by it in his weakened state. Afterwards, Brain questions Darkness as to why it had chosen Ventus and its relation to the Master of Masters, before summoning his Keyblade and battling Darkness alongside Ephemer and Skuld.
With most of the leaders defeated Darkness knows it's on the verge of victory and yet was coaxed by Brain into revealing what it really wanted and it says infection. Upon clarifying the entity said it was just doing it's role based on the book of prophecies, and used Ventus to separate itself from the hive mind of Darkness giving said individual a will of it's own not caring what sacrifice was needed. When Player and the Union Leaders rise up to fight again Darkness still remained confident they would lose and as Lauriam spoke of getting out the entity scoffed at them, asking how they would accomplish it. Hearing of them planning to use the lifeboat as Maleficient had the entity ridiculed them for their cluelessness explaining the true purpose of the lifeboat and how with Maleficient's departure, the lockdown and destruction of the data world had already started. Despite them briefly defeating the entity, it was unable to be destroyed with no form. Afterwards Ventus awakens using the light within him to dispel the creatures of darkness. Aware that it used him to give itself a form, Ventus knew could do the same turning the entity into something they can destroy. The entity fights off with him alone.
Meanwhile, in the real Daybreak Town, another Darkness speaks with Luxu regarding his and Master of Masters' plans and the whereabouts of remaining four Darknesses. Shortly after, the remaining four Darknesses appear to Player, Ephemer and Skuld in the Data Daybreak Town and try to make them open the gate to Central Game Station so that they can spread to other Data Worlds. Player appears to be possessed by Darkness and attacks Ephemer and Skuld and defeats them both in battle. Player then moves to kill Skuld but Ephemer shoots a beam of energy through Player, sending them and the four Darknesses into the gate leading to the Central Game Station while locking the gate from his own side. In the conduit between the Data Daybreak Town and Game Central Station, Darkness says they should dispose of Player's body as they have no use for them anymore but Player reveals they were pretending to be possessed the entire time, tricking the Darknesses. Player then seals the gate from their side trapping the four Darknesses in the conduit between the Data Daybreak Town and the Central Game Station which enrages the Darknesses.
Ventus soon after travels into the future, carrying one of Darknesses deep inside his heart.
Kingdom Hearts III Re Mind
When Sora begins his journey to restore Kairi's heart using the power of waking, he first enters Ventus's heart. When attempting to wake Ventus, he is stopped by an entity referring to itself as "Darkness". Sora questions if "Darkness" is Vanitas based on the latter's last words, but does not get a reply as the entity forms itself into the Dark Inferno χ and battles Sora.
True to its name, Darkness is a miasma of dark energy. Despite claiming to be formless, a humanoid shape with no definable features can be made out within the miasma.
Not much is known about Darkness. According to the Master of Masters, Darkness has no sense of reason and is difficult to understand because its thought processes are completely alien to humans. Darkness does not understand humans aside from their negative emotions, and hates beings with hearts of light.
Darkness has the ability to possess, control and hide inside of other beings, such as when they possess Ventus. As shown when they impersonate Ava, the Darkness is able to transform their appearance. It can manifest as other forms of darkness, such as Heartless. It also has the ability to go invisible, allowing its actions go unnoticed by others. The Darkness can increase their numbers by indirectly influence the negative thoughts of other people, allowing their hearts to succumb to darkness and giving birth to the Heartless.