Before Kingdom Hearts Union χ
Long ago, 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.
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.
Darkness is true to its name being simply a miasma of dark energy. Despite claiming to be formless, a humanoid shape with no indefinable features can be made out within the miasma.
Not much is known about Darkness. According to the Master of Masters, he describes Darkness has no sense of reason and is difficult to understand because its thought processes are completely alien to humans. Conversely, Darkness does not understand humans aside from their negative emotions. Darkness also 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.