The Heartless Apprentices are a group of villains originating from the World and the major antagonists of The Interference and its sequels, technically starting from The Interference II: Curse of the Crimson Elixir. They are the collective Heartless of the apprentices of Ansem the Wise.
As a group, the apprentices - Xehanort, Dilan, Aeleus, Even, Ienzo and Braig - met under service to Ansem the Wise several years ago. Ansem was performing experiments to discover the nature of the heart, and Xehanort offered himself as a test subject, to which Ansem agreed. However, Xehanort was more interested in performing his own experiments.
The apprentices kidnapped people from around Radiant Garden and brought them underneath the castle, where they performed many dark experiments on their hearts. Eventually, the experiments caused the victims to lose their hearts. Xehanort found the remaining dark creatures and became intrigued, naming them 'Heartless'.
Secret research into the Heartless took place, concerning their ability to consume hearts and change them into more Heartless. In turn, the apprentices began to accept more and more darkness into their hearts, which caused some kind of physical deformations, causing Ansem to notice their change and reprimand them. Despite this, Xehanort performed more and more research, accidentally destroying the barriers between the worlds in the process and allowing King Mickey to meet Ansem.
Through his research, Xehanort became successful in producing artificial Heartless, which he marked with an emblem to distinguish them. However, Ansem finally discovered the experiments and put an end to the apprentices' work, forcing them to seal off the laboratory and discontinue their work. In response, Xehanort banished Ansem to the Realm of Darkness and began his takeover and destruction of Radiant Garden, with his research also revealing the existence of Nobodies.
Eventually the apprentices all gave themselves to the darkness in order to control the Heartless. However, only Xehanort's Heartless became active, while the others lay in waiting for more than a decade, only resurfacing upon the World's Interference by Alexander Karsath.
Despite being referred to as a group, each of the apprentices is a free agent, aiming to defeat Alex on their own, although they remain allied with each other. With every interference, one of the apprentices moves into the newly-created world and befriends its current villain, offering their services and knowledge of Alex's advantages in exchange for a share of power.
Xehanort is the only member of the group whose Heartless form is no longer alive by the time of The Interference. He is the final boss of Kingdom Hearts, taking the name Ansem. Sora defeated him by opening the door to Kingdom Hearts. His Nobody, Xemnas, is the main antagonist of The Interference.
Dilan is an antagonist in The Interference II: Curse of the Crimson Elixir. He allies with Jack Crowley, the villain and final boss of the original game. During the course of the story, Dilan convinces Erica Karsath to become subservient to him and accept her own darkness, pitting her and her brother against one another. When Crowley absorbs the Crimson Elixir, Dilan also uses him to create a Guardian Heartless.
Aeleus is an antagonist in The Interference III: Wings of Time. He assists the monster Lavos in causing the apocalyptic events of 1999AD, and actually succeeds in killing Alex using Lavos, though he is brought back to life. He resides in a pocket dimension outside of Guardia's timeline until his defeat. During the story, his name is spelled as 'Elaeus', which was a mistranslation found in Kingdom Hearts II.
Even is an antagonist in The Interference IV: Dark Renegade, where he serves Baron Praxis. After the fall of Dead Town, Even is present for Alex's Dark Eco treatments, taking over the experiments personally. He manages to retrieve Joumae from Alex's heart and appropriates him as his own apprentice.
Ienzo makes a cameo in The Interference IV: Dark Renegade visiting Even on Alex's seventeenth birthday. According to Braig in The Interference: Trials of Remembrance, Ienzo is in 'an awkward place'.
Braig makes a cameo in The Interference IV: Dark Renegade along with Ienzo, visiting Even. Directly after the meeting, according to Trials of Remembrance, he realised that working for Master Xehanort doomed every other member of the group, and merged with his Nobody Xigbar.