Malachi, also known as Sheidheda (Trigedasleng for "Dark Commander")

Malachi was a Commander, given the title Sheidheda because of his dark history, and was a member of Sangedakru. 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.

Madi is communing with the other Commanders including Sheidheda. Gaia mentions to Josephine Lightbourne (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.

Overall, Sheidheda is nothing more than an evil, bloodthirsty, sadistic commander and warlord, whose favorite saying is "kneel or die." Before his death, Sheidheda was a fearsome tyrant who killed his own teachers for binding him

Sheidheda repeatedly appears to Madi and guides her towards a path of violence, eventually physically possessing her body

You run across the girl possessed by the Dark Commander and she says You kneel or die!