The Shadow Knights are a large group of antagonists in the series; they work for the Shadow Lord in the Nether and they will do anything to get people on their side.
About 15 years later, as of Season 2, the Shadow Knights seem to have become stronger and have built a nether portal somewhere in the world from which they are sending Shadow Souls into the world. We find out in the end of Season 2 Episode 36 that the portal of the Southern Wolf Tribe has somehow been opened by Zenix, another Shadow Knight.
In Season 3, due to the return of Shad, the Shadow Knights are even more active, now led by Gene to find Zane and kill him for stealing the fragment and also find Alina as she holds Shad's relic and kill her mother, Lady Irene, aka Aphmau.
Shadow Knights are mostly created when someone dies or is stuck in the Nether, but some are created in the Nether. They undergo mental change, with other Shadow Knights convincing them that they were abandoned.
To achieve immortality and enhanced powers, a turned Shadow Knight must go to the overworld and kill someone they were once close to. A shadow knight who has not done this yet is called a "premature shadow knight". However, all Shadow Knights are immortal if they stay in the Nether (except Zenix apparently by passes this and absorbs the others somehow).
All Shadow Knights, even premature ones, can transform into a larger version of themselves that is more powerful, and has armor and weapons.
A method for curing a Shadow Knight has not been found. However, knights like Sasha, Laurance, Vincent, and Vylad etc, have been able to keep control of themselves through mind over matter.
Shadow Knights develop more feelings when being in the overworld for to long (ex: Laurance has developed feelings for Aphmau, although Laurance already had feelings for Aphmau in the beginning before he turned; another example is that Vylad remembered Garroth)
The main goal of the Shadow Knights seems to be helping the Shadow King, Shad the destroyer, rise to power in the Overworld once again, although their original motives are vague. They seem to want the Lord system gone, which may be the reason why a Shadow Knight must kill their past Lord. But another reason why they kill their old Lord's is because to obtain immortality is to kill someone you cared for, and most of the Shadow Knights are guards and guards protect and care for their Lords. Therefore another reason for killing their past Lord besides the one above.
As of Season 3, it may be possible that the Shadow Knights are also planning to not only take over the Overworld, but also kill Aphmau as she is known now as Lady Irene and take her daughter, Alina. This is supported by Gene that Aphmau must be dead and Alina be taken and that also Zane must be killed for stealing Shad's relic.
Although they have appeared differently and each have their own face, the army usually has the same red armor and red helmet on. However, characters like Sasha and Vincent have never been seen wearing this armor. Most use the same red sword. The armor looks like it has black bands on it also. If they transform, red particles will be seen circling them with eyes of unnatural color like red and purple.
- Laurance ~ While he is technically a Shadow Knight, he held onto himself by thinking about Aphmau while undergoing the transformation but he has transformed into his shadow knight form a couple of times. Laurance was thought to have become evil after the events of late Season 2 and in the Season 3 trailer, but was proven wrong as Laurance tries to hold back Gene when Aphmau, Katelyn, and Garroth and Zane are in the Nether. Even though he now resides to the Nether, Laurance led a rebellion against Shad, along with Zenix. Even now, it seems he is not in stable minds as he constantly has human emotions and Shadow knight emotions. He has not been able to control his Shadow Knight transformation either. Although he is physically strong, but he is mentally weak, as stated by Vincent. It is unknown who's side he is truly on, as he constantly changes to Aphmau to the Nether due to his inability to control his Shadow as of late.
- Vylad ~ Appearing evil within the first few episodes, he soon turns against the Shadow Knights despite being one for a while. It has been confirmed that he is Garroth and Zane's younger brother. He is responsible for Levin ending up with Aphmau. He was the reason why we have Aphmau now, also known as Lady Irene, who woke from her long slumber after Vylad called her using the staff given to him by Hyria, just to save his half brother, Garroth.
- Vincent ~ He is a full Shadow Knight and has been for the past 100 years. He is currently the second in command guard or third wheel of New Meteli.
- Amethyst ~ A reformed Shadow Knight in Pikoro Village.
- Sasha ~- Kenmur's old fiancee, left behind in the Nether, thought to be dead. Gene turned her into a Shadow Knight. In Season 2, she was found by Aphmau and taken in to the cell in Phoenix Drop, when the Phoenix Capital was found, they moved her there. At the end of season 2 though, it is found that she and Michi had been freed by Laurance when he was not in a fully stable mind.
- Alexis ~ Alexis was turned into a Shadow Knight by Zane when she was a baby. Later, Laurance stated that she wasn't a proper Shadow Knight due to the means she was turned, suggesting she was not as strong as even a premature Shadow Knight. In Season 2, there are no signs that indicate that she is still a Shadow Knight.
- Gene ~ Dante's brother. Turned into a Shadow Knight at a young age and destroyed his village, ruining everyone but Dante's memories. He is now a very important part of the Shadow Knights because of his ability to alter memories.
- Zenix ~ became a full knight a long time ago. Found by Garroth and became his apprentice. Has been conspiring against other Shadow Knights to raise the Shadow Lord and control him. Goes in the Nether in Season 2 and consumes other Shadow Knights for more power. In season 3, he seems not to be as insane being seen with Laurance. It is unknown if he is now good or is still bad. He leads a rebellion against Shad with Laurance.
- The Shadow King ~ leader of the Shadow Knights who wants to take over the world. He is now looking for Aphmau and her daughter as he wants his relic back from them in order to help conquer the world.
- Glenda ~ Attempted to sacrifice Cadenza to open a nether portal. Deceased as Aphmau had killed her when saving Cadenza.
- Zane thought to be to dead but soon after came out of the Nether in Bright Port wearing what seem's to be the new Shadow Knight outfit. He kidnaps Emmalyn and forces her to research for information about Shad, the Shadow Lord, and his fragment, while trying to get away from other Knights who were sent to kill him.