The Next-Level Villains is the term that the Police Force created to name a group of hugely transformed villains. They are the successors of the Impromptu Villains, the next step in the mysterious objective of the Villain Factory. They are from the spin-off series Vigilante - My Hero Academia: Illegals
The Next-Level Villains are people who are taken by Villain Factory, and all of them had previously been Trigger consumers and turned into Impromptu Villains. After distributing the drug through the city and collecting data of the Impromptu Villains, the criminal organization studies the capacities of each of them, and if they consider that some of the users have potential, their agents kidnap them and take them to their secret laboratory in Onomura Pharma Corp.
Once there, the bodies of the Trigger users are modified via bio-engineering into giant, powerful, monstrous beings, to strengthen their Quirk and allow them to be able to take higher doses of Trigger than before. Once the process of transformation is finished, the Villain Factory dope them with Trigger and releases the newly created Next-Level Villain through the streets of Naruhata where they inevitably go on a rampage, attacking everyone and destroying everything in their way.
The Villain Factory does not do this just to cause chaos. Inevitably, the violent actions of the Next-Level Villain attract the attention of the Heroes and / or the Vigilantes, who will face them to stop them. The Villain Factory doesn't care if their creations end up being defeated and imprisoned, the only thing that interests their mysterious leader is to collect combat data, and if the Next-Level Villain turns out to be a promising one, also acquire samples of them.
Unlike what happens to Impromptu Villains once there are defeated and Trigger doses stops being effective, the physical transformations of the Next-Level Villains is permanent, that is, they do not recover their original bodies once the Trigger's effect is over. When that happens, they regain their senses, but their power fades without more Trigger doses.
The first time one of these villains appears is when the criminal Kuin Hachisuka decides to use Teruo Unagisawa, a boy with an Electric Eel Quirk that he had already been arrested for using Trigger and becoming a Impromptu Villains. She wants to use him to sabotage the performance at the roof of Marukane Department Store, and then inject Trigger to several spectators to turn them into Impromptu Villains, and thus cause even more damage. However, her plan fails when she is defeated by Knuckleduster, while Teruo is defeated by Shota Aizawa, and taken into custody by the police.
Days later, the Factory releases Kirihito Kamachi, a person with a Mantis Quirk. A vigilante, The Crawler, tries to stop him, but it turns out to be too powerful for him and is forced to retreat. Then, Kirihito is easily defeated by the hero Captain Celebrity.
Days after the arrest of Teruo, the detective Naomasa Tsukauchi has a conversation with Shota and Nemuri Kayama to talk about the new villains that had begun appearing throughout the city. He tells them that Teruo had been arrested twice for being an "Instant Villain", however he had undergone a drastic change between his first and second arrests in that his physical form had completely changed. In addition, the police had found several other villains with similar alterations that had concentrations of Trigger present in them which was several grades higher than the lethal dose of the drug. This had led the Police Force to suspect that someone had purposefully altered their bodies to withstand higher doses of Trigger.
Naomasa tells them that he believes that the Villain Factory, responsible for the distribution of Trigger, is also administering to others and altering them to be able to withstand greater amounts of the drug. Shota says that the organization must have some greater goal in mind because by themselves their "Instant Villains" and "Next-level Villains" could not do much and were more like disposable pawns.
Elsewhere, the Scarred Man, one of the Villain Factory agent, release another Next-Level villain, while Captain Celebrity and Tensei Iida finish shooting their commercial. This foe is Bat Villain, a previous Instant Villain who was arrested before by Tensei with the help of Vigilantes of Naruhata. Bat Villain wants revenge on those who locked him up and attacks Tensei before going in search of Koichi and Knuckleduster. Fortunately Tensei emerges unharmed from the attack and Captain Celebrity pursues the criminal. Bat Villain locates Koichi and makes his way towards his location. However, Captain Celebrity finally managed to catch up to him as he slowed down to pick Koichi off and knocks him out while also saving Koichi.
Some time later, The Scarred Man decides release another Next-Level Villain, Octoid. This foe proves to be be a tough nut to crack, even for the experienced Eraser Head, who needs help from Koichi, the Hotta brothers, and Kirihito Kamachi to capture him. Even so, Octoid manages to escape, finding Scarred Man in an alley ,who had been sent by the leader of the Villain Factory to extract a sample from him. Scarred Man easily defeats the Next-Level Villain and fulfills his objective, and when he is arranged to kill Octoid, Koichi, Aizawa and the rest arrive to where he is, forcing him to fly away.
After this, The Scarred Man has the objective to kill Captain Celebrity, and for that he decides to use a new type of Next-Level Villain: The Bombers. After a Christmas party, a Bomber takes a taxi with the Little Sisters of Saint Lila's Academy inside. Captain Celebrity goes out in pursuit, but just when he is going to reach them, the Scarred Man blows up the creature with a device. The girls and Captain Celebrity survives the explosion, thanks to his aerodynamic barrier, but the Scarred Man manages to deduce the weakness of Captain Celebrity's Quirk, and decides to prepare a new attack to end his life, releasing more flying bomber.
The scarred Man attacks again during the Captain's farewell party at the Tokyo Sky Egg. Previously, the laboratory where the Next Level Villains are produced had been assaulted by the police, but the Villain manages to escape along with five bombers after blowing up the place. Then, they reach the Sky Egg and start the assassination plan. The Scarred Man was about to achieve his goal, but the intervention of Koichi, several heroes and finally All Might makes him fails.