The Mycelium, also known as The Mother, is a seemingly sentient and powerful wall, found in Fort Salem, with some fungal properties. Both Sarah Alder and Izadora refer to this with a "her" pronoun, refusing to use "it" in reverence of the power they detect from her.
Little was known about the Mycelium at the beginning of the series. Izadora still did not fully know much despite studying her for years.
During Raelle Collar's basic training at the Fort, Raelle wandered into the Necro tomb that housed the mycelium and was drawn to her. The mycelium also reached out, using her mycelial threads to form a hand and touched Raelle's extended finger, Raelle is apparently so far the only known person to touch the Mycelium without major harm to her body. A connection has since formed between the two, unbeknownst to Raelle.
When Raelle was stabbed in the chest halfway across the globe, the Mycelium, with their connection, protected Raelle and caused an explosion that somehow healed her. The blast remained active for a time even as Raelle, along with Abigail Bellweather, moved. During Raelle and Abigail's march through the field after the explosion, mushrooms grew and spread through any land touched by the resulting blast, and even humans in the area were colonized by her. Initially mistaken to be due to Raelle and Abigail's link, Izadora later figured out that it was due to Raelle's connection to the Mycelium.
When a severed head that had been colonized by the mycelium was brought back from the blast zone, Izadora took off the goggles off of the head, allowing the fungi to spread over the eyes. The head started talking, taking on Raelle's voice and mimicking the exact words she was speaking at the time. Izadora and another Necro specialist studied it and saw that the brain was now made up of the same exact properties as the mycelium. Having realized the connection, Izadora pretended to be aiming to kill Raelle — now perceived by both Raelle and the mycelium to be a true threat compared to their previous tests — and the mycelium protected her by wrapping her in a gelatinous cocoon.
Prompted to try to use song seeds that will trigger the Mycelium's power in her without the threat of death, Raelle tried to sing a seed she had previously seen Scylla Ramshorn use on a death cap. The Mycelium reacted, her filaments lashing out and into Raelle's visible orifices, stopped only by Izadora. The Mycelium had now successfully colonized her larynx with mycelial threads - fundamentally changing Raelle's vocal folds, ventricular folds, as well as her epiglottis and epiglottal fold. After this incident, Raelle can now effortlessly speak fluent Mothertongue and became capable of using a complex technique with her throat, able to sing seeds with "thousands of layered voices" for increased strength of whatever seed she sings, even allowing her to break through walls and thick layers of cement.
In Mother of All, Mother of None, Raelle is once again drawn to the Mycelium. Once approached, she takes the shape of a hand and face belonging to Raelle's mother, Willa Collar. Willa brings Raelle into a vision where she reveals how the Mycelium came to be. The Mycelium is discovered to be made out of the bodies and souls of the dead witches who were the first coven of Sarah Alder, after their death at the hand of the The Camarilla while Alder was busy pushing back enemy forces. They became the Mycelium after Sarah sang in mourning - their bodies sinking into the earth and becoming the Mycelium. According to Willa, she is where all witches go when they die (at least the "strong" part of them, such as Willa's love for her daughter). Sarah built Fort Salem on the ground the witches died on as a way of always remembering her fallen coven.
The Mycelium is a fungal mass similar to death cap when growing. She is actually a hive mind, composed of the many witches who died, and possesses a personality on her own. She is driven by the single goal of destroying the Camarilla. The Mycelium can transmute living matter into mushrooms on a large scale, and can form a connection with a witch after colonizing her vocal cords like a parasite.
The witch who is connected to the Mycelium can summon vast amount of spores, destroy inorganic matter, and transmute living matter into fungi like the Mycelium herself. The Mycelium also amplifies the witch abilities of her host; she improved Raelle's fixing work and protected her from the injurious side effects of the Christo-pagan workings. Because the Mycelium is made from the bodies and souls of dead witches, a more thorough colonization by Mycelium will layer thousands of witch voices upon the host's own voice - further enhancing the witch's abilities and granting the host the ability to effortless use mothertongue.
Possessing a will of her own, the Mycelium can manifest and act without the intervention of her "host" - allowing her to protect her dying host from nearby threats and resurrect her. While she can kill humans, she does not attack witches. Her power is strong enough to cure weaker strains of the witchplague and slow down the progress of stronger strains. The usage of the Mycelium power requires to use a special seed sound normally used by Necro at Fort Salem to grow death cap on recently deceased creature.
During the resurrection process while the host is clinically dead, the host can experience a "death dream" in a place filled with Mycelium where she can communicate with the Mycelium.
- Fort Salem is referred to as a "sacred ground" when it comes to the night of Samhain. The fact that dead witches can be summoned may be related to the presence of the Mycelium, supposedly the place where all witches go when they die.
- The burst of spores created by the Mycelium is referred to as the "witchbomb".