He was a Commander, given the title Sheidheda because of his dark history. He killed his own Flamekeepers and has been using the Flame to convince new Commanders to turn evil and to kill people, including their own teachers. To help newly-chosen Commanders from being controlled by Sheidheda, the Flamekeepers came up with the "Separation Ritual", during which the Commander calls upon the past Commanders to help stop Sheidheda.
Sheidheda was born a Nightblood and eventually became a Commander of the Grounders. He was also Sangedakru's greatest champion. Instead of following the teachings of his Flamekeeper, he felt like the Flamekeeper was trying to control him. He eventually became impatient and killed his own Flamekeeper. He ultimately killed three Flamekeepers before being killed himself by his fourth.
During Indra's childhood, he was the Commander and took Trikru land, he butchered anyone who wouldn't bow and their families. She also saw him.
In The Children of Gabriel,Madi mentions to Gaia that while listening to the past commanders, she has been hearing a commander who scares her. Gaia reveals that it is Sheidheda - the Dark Commander and asks Madi to avoid him.
In Memento Mori, Madi is communing with the other commanders including Sheidheda. Gaia mentions to Josephine (pretending to be Clarke) that Sheidheda is getting too strong and they need to perform the Separation Ritual to stop him. Josie ignores Gaia's advice and wakes Madi up abruptly. After that, the Dark Commander starts revealing himself to Madi whenever she looks in the mirrors. After realizing what's going on, Gaia tells Madi that they must perform the Separation Ritual right away in order to call upon the other commanders to stop Sheidheda.
During the ritual, Madi finds herself in a room with no one but Sheidheda. Sheidheda tells Madi that Gaia is lying to her in order to restrict her. He tells her that his own teacher used to control him too so, he killed him. He urges Madi to kill Gaia. Madi refuses and wakes up. Later on, after learning that Clarke is dead, Sheidheda convinces Madi that Gaia is to blame for Clarke's death. He asks Madi to kill Gaia but Madi banishes her instead. Taking advantage of Madi's anger and grief, Sheidheda agrees to help guide Madi in her quest to kill the Primes.
In The Old Man and the Anomaly, Madi sees Sheidheda as she discusses killing the Primes with her friends and effectively committing genocide upon Sanctum. When they refuse to help her, Madi enacts her plan on her own under Sheidheda's influence.
In Matryoshka, Sheidheda continues to influence Madi to madness to the point that Gaia suggests killing Madi to prevent Sheidheda from being loosed upon the world which would make them long for the days of Blodreina. As the others try to get answers from Madi, Sheidheda orders her to tell them nothing. Raven Reyes realizes that Sheidheda managed to isolate the spirits of the other Commanders, somehow manipulating the AI inside the Flame to do it. Raven suggests that this means that the code from the minds of the Commanders can be deleted and they can possibly get rid of Sheidheda for good. Raven orders Gaia to go get get Becca's book to see if her theory is correct.
In Ashes to Ashes, while Madi is tied up to withdraw bone marrow for Nightblood serum, Sheidheda wants her to kill Jackson but Madi resists. While playing chess with Madi in the Flame's mindspace, Sheidheda insists that Madi should kill everyone in Sanctum and rule over this new world just like he ruled over his world (Earth). Madi doesn't want to kill anyone else besides the Primes but Sheidheda insists they should kill anyone who betrays them or stands in their way in getting revenge for Clarke. Madi agrees and asks him what she needs to do next.
In Adjustment Protocol, Raven tries to delete Sheidheda from the Flame without the help of Becca's notebook. During the process, Sheidheda briefly possesses Madi and threatens to kill her if he is deleted. Raven proves unable to hack the AI on her own and delete Sheidheda. Raven notes and is confused by the fact that the AI seems to be protecting Sheidheda.
In The Blood of Sanctum, Sheidheda possesses Madi and uses her to order the awake Wonkru members attack the Primes and their guards. Several people are killed before Clarke knocks Madi unconscious. Using Becca's notebook, Raven successfully hacks the AI with the plan to isolate Sheidheda, move him to another computer and then delete the Dark Commander for good. However, she finds a kill code that wasn't in the notebook, one that would destroy the Flame and realizes that Sheidheda created it and is forcing them to choose.
After Clarke floats the other Primes, Russell intends to execute Madi, but is talked out of it by Sheidheda who offers him a better way to get revenge. Using Madi's command of Wonkru, Sheidheda and Russell turn them against Clarke and the others. However, Clarke manages to break through to Madi who regains control from Sheidheda and orders Wonkru to take Russell and his men captive.
Enraged, Sheidheda strangles Madi in her mind while she struggles against him to no avail. With no other choice, Raven activates the kill code, deleting the Flame and causing Sheidheda to disappear. Moments after the dead Flame is removed from Madi, the group discovers that Sheidheda tricked them and his consciousness is uploaded to an unknown location.
In From the Ashes, Raven is concerned by her inability to trace Sheidheda's transmission. After Madi starts drawing memories that are not her own, Clarke questions if they are Sheidheda's, but Gaia suggests that they might belong to one or all of the other Commanders instead.
After Clarke knocks Russell Lightbourne unconscious, he awakens in a Mindspace resembling the room Sheidheda and Madi met in. Sheidheda, having downloaded himself into Russell's Mind Drive, comments that he likes Russell's current host better than his original body. Stating that he is now Russell, Sheidheda slits the Prime's throat from behind, killing him.
Sheidheda regains consciousness, now fully resurrected in Russell's final host. By playing on Clarke's emotions, he is able to get her to save "Russell" from the fire engulfing the Sanctum palace. However, as Sheidheda revels in his resurrection, Clarke announces that Russell Prime will be executed for his crimes the next day.
In False Gods, Sheidheda, still posing as Russell, is mentioned to have been visited by many of Russell's remaining followers. Looking through a book, Sheidheda enjoys the sensations of having a body again before being visited by Clarke and Indra who announce his execution at sunset. Given a choice of executions, Sheidheda decides on burning at the stake so that Russell's followers will be forced to witness it as will Clarke.
Later, concerned that the Adjusters are planning something drastic, Jordan Green visits "Russell" for help. Sheidheda rejects food, stating that it will all turn to ash anyway and wanting to die and facing it are two separate things. Jordan begs Sheidheda to help "his" people who are planning some kind of an attack to stop the execution which will lead to people dying. Sheidheda simply tells Jordan to tell them not to kill him, surprising Jordan by his lack of caring. Jordan tries to convince Sheidheda to go out doing the right thing, using his voice to stop his people from dying. Sheidheda suggests that he ask his people nicely and Jordan says that he won't be asking by his father's son before leaving.
Jordan convinces the others to bring "Russell" out to address his people, though Indra threatens to kill him herself if he doesn't say what he's supposed to say which Clarke warns Sheidheda. Sheidheda states that they both want the same thing, peace before addressing the crowd. Sheidheda tells the crowd that if his death is needed to bring peace, then so be it and urges them not to take action as Clarke is right that they are all that's left of the human race. Declaring Sheidheda to be a false god, a man tries to kill him, but only succeeds in shooting Sheidheda in the shoulder before being beaten down by the crowd. As chaos breaks out, Clarke and Eric Jackson tend to Sheidheda's gunshot wound which damaged his subclavian artery. As he's taken off to Medical, Indra asks if Sheidheda will survive. Clarke believes so and Indra states that she hopes he does otherwise "Russell" will become a martyr to the fanatics and without Wonkru to keep the peace, Sanctum will explode. Clarke reluctantly realizes that they can't kill him which Indra confirms.
As Jackson treats his wound, Sheidheda questions if something is wrong. Jackson asks if he even cares about the people he's killed and Sheidheda states that his list of regrets is a long one before being interrupted by Indra. Indra informs Sheidheda that his execution has been cancelled as Russell's followers will burn Sanctum to the ground if they go through with it. The Dark Commander knows that they don't have the forces to stop it, pointing out that some of his guards are gone. One of Russell's followers warned of the attack and so is allowed to see Sheidheda as the fanatics no longer trust Jordan. The woman secretly tells Sheidheda that he had been right about Jordan who never suspected and did exactly what Sheidheda said he would while the shooter, Tobin, is dead. Pleased, Sheidheda enjoys the cookie she brought as the woman leaves.
He primarly wears a hood to hide his facial scars. He only has one eye on the right side of his face. His left eye is covered by heavily scarred skin. He is a pale blond man who keep his hair in a manbun.
Following his resurrection in Russell Lightbourne's final host body, Sheidheda takes on the form of Russell himself.
His first teacher chained him to a wall, which made him kill that teacher.
He was a feared ruler of Polis during his lifetime as he wants Madi to kill others such as Gaia since his victim record includes his flamekeepers/teachers.
According to Gaia, if he is able to successfully use Madi to continue his reign of terror, it will be much worse than even Blodreina.
He is the first male commander shown in the series.
Jason Rothenberg explained that when Sheidheda uploaded himself when the Flame was destroyed, the code went into the ether of the system WiFi on the ship and found its way into Russell's Mind Drive.
The reason that it took so long for him to take control is that Russell had to be asleep or unconscious for them to be in the Mindspace together as seen with Clarke and Josephine. Russell had been refusing to eat, drink or sleep since his arrest.
In a Trigedasleng conversation that did not make it into the finished episode, Sheidheda mentions waking Sangedakru, which may hint at his original clan.
In "False Gods," it is revealed that Sheidheda was a member of Sangedakru. Knight, a member of Sangedakru, says Sheidheda was the clan's greatest champion.