"Kushagara" (くしゃがら) is a word used to refer to the main antagonist of the short story Kushagara, that is part of the Thus Spoke Kishibe Rohan: Short Story Collection. As a taboo word, the actual name of the creature cannot be published anywhere, so the story refers to it as "Kushagara" as a placeholder.
Kushagara is a supernatural creature akin to a parasite that manipulates a person's curiosity to take control of their bodies, forcing them to be obsessed with the forbidden word.
Kushagara is a bizarre-looking monster that lurks inside a host's throat and makes Rohan feel disgusted. In the live-action adaptation, it appears as multiple slimy, dark worm-like creatures. When Heaven's Door is active, it appears as a dark pouch like a bound-pocket book, formed by the ends of two pages stuck together. The pouch grows as time passes, as a shopkeeper had a smaller and slimmer pouch compared to Jugo's. The outside of the pouch has innumerable small handprints resembling a messy pattern etched into the page, enough to turn it black. The creature stays inside the pouch until it is ready to escape.
Kushagara has a strange voice that constantly repeats its name, and the pouch it's inside squirms every time it speaks. In the live-action, the voice has alternating low and high pitches making it sound like multiple creatures. It is manipulative and acts selfishly, as its primary goal seems to be reproduction without considering its host's body. It forces humans into compulsively researching or repeating its name to propagate itself to others. After a month passes when its host reaches the peak of its obsession, the creature tries to escape from their body, restlessly squirming from within the pouch and pulling the closed part of the pouch from within to rip it open.
The Kushagara creature is an information hazard as it spreads from one person to another by stimulating one's curiosity when they discover that it is a taboo word, and then infecting susceptible hosts. It takes control of a person like a parasite or germ, driving them to only think of the word. This causes personality changes in their host, as they become more aggressive over time. Their host will no longer be able to sleep or accomplish anything else as their thoughts are invaded with trying to understand the word "kushagara". However, since it's a taboo word, their host will never discover its meaning as it cannot be recorded anywhere. This also makes it a natural counter to Heaven's Door, as Rohan cannot write that a host would forget the word since it would just get automatically erased. It is unknown what would happen to its host if it managed to break free from its pouch, though Rohan wonders if it's similar to some species of parasites that eat their host from the inside out once they're done growing.
It is unknown how the creature formed, but at some point, it seemingly was controlling a mysterious man who poses as the new editor of Jugo Shishi after Jugo's previous editor was promoted. The fake editor sends Jugo a list of prohibited words in manga, including the word "kushagara". When Jugo asks about the meaning of the word, he says that since the use of the word is forbidden, knowing its meaning isn't necessary. The parasite transfers from the editor to Jugo, who compulsively tries to figure out what the word means.
In the live-action TV drama adaptation, Jugo's ordinary editor nearly falls victim to Kushagara instead. He was affected by the word but couldn't find its meaning. To relieve himself of his nagging curiosity, he passed on the word to Jugo in a list of forbidden words and then secluded himself from the world in a warehouse near where he lived, telling his parents not to tell anyone. However, Kyoka Izumi finds him. Although he's not overtaken by Kushagara, he's become somewhat unstable in his efforts to forget the word.
Jugo greets Rohan Kishibe, who he shows the list of prohibited words. He makes a once-in-a-lifetime request to Rohan: if Rohan ever learns anything about Kushagara, he should share it with him. Jugo descends to madness as a month passes by of him dedicating his days to researching the word's meaning, spreading the word to various other people such as a used bookstore's shopkeeper. When Rohan reunites with him, he recognizes that Jugo is unstable. Jugo reveals that even if he tries to forget about Kushagara, he thinks about it so much all the time that he can't even sleep. Rohan gets quiet and Jugo increasingly starts mentioning Kushagara in all of his sentences.
Rohan sees the creature poking from the back of Jugo's throat and uses Heaven's Door. The creature attempts to tear through the pouch in Jugo's pages but Rohan manages to make it stop by writing in a command that Jugo would forget everything that happened in the past month. For the shopkeeper, Rohan writes that he would forget everything from the past few days. Although that resolved their situations as they forgot about the word, Rohan is unaware whether the pouch still exists within them and wonders whether he has one in himself as well.