A woman recently sent an absurd and disturbing letter to The Cut's "Ask Polly" column.

She said her in-laws refused to stop serving her mushrooms even though she is deathly allergic to them.

To make matters worse, her partner told her they were never really that keen on mushrooms before, she said.

In response, Heather Havrilesky said they were "truly terrible human beings" and "next-level, off-the-charts batshit."

Shannon Thomas, a trauma therapist, told INSIDER that the cruelty shown by toxic in-laws is "like narcissism on steroids."

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Spare a thought for the woman who recently wrote to Heather Havrilesky of The Cut's "Ask Polly" column with the most chilling and absurd story about her partner's parents.

And as you read, know that there might be more than ignorance at play here. According to a trauma therapist, it's possible the in-laws are toxic narcissists, hellbent on destroying their son's wife.

No matter how bad you might think your own in-laws are, this letter might be worse than you could imagine. It details her husband's parents literally toying with her life because of an apparently newfound obsession with mushrooms.

The woman, who signed the letter "Disrespected Daughter-in-Law," started by explaining that she is deathly allergic to mushrooms. She said she carries an EpiPen and had been hospitalized multiple times after accidentally coming into contact with them.

She said her in-laws were aware of this but for some reason can't stop serving up mushroom dishes. She said they'd even gone so far as adding mushroom powder to mashed potatoes, which nobody has done in the history of cuisine.

They apparently find a way to add mushrooms to almost everything.

"One time, they made a point to make a special plate of mushrooms and pass it around," the woman wrote. "While my mother-in-law said, very rudely, 'I would've liked to add mushrooms directly to the salad, but SOMEBODY has problems with it!'"

To make matters even weirder, the author's husband said his parents never really cooked with mushrooms before, she said. And now that they've seemingly developed a taste for them, they refuse to give up fungi.

The woman wrote that when confronted, her father-in-law said, "We're not changing what we eat for one person."

Read more: Narcissistic parents identify their children as either a favorite or a scapegoat, and they pit them against each other

In response, Havrilesky said the in-laws were "truly terrible human beings."

"Your in-laws are next-level, off-the-charts batshit," she said.

She offered up one explanation: Sometimes a group of people assume the traits and behaviors of a sociopath and get worse and worse through confusing and gaslighting behaviors.

Jane Fonda and Jennifer Lopez in "Monster In Law." Monster in Law/New Line Cinema

"Because these people are confused and weak and angry — and because they're rendered increasingly more confused, weak, and angry by their exposure to each other — they tend to have less and less contact with those outside the group," Havrilesky said.

"And when they do encounter someone who's not in the fold, they recoil and attack. Anyone who questions the group is attacked with words and actions. Anyone who questions the group is bad, and the group is good."

She added that it doesn't really matter either way, telling the woman that "they're literally trying to kill you."

"How is it possible that they're all engaged in this charade of loving the ever-living hell out of mushrooms out of nowhere, in spite of the fact that they know you could die if you eat one?" Havrilesky said. "What on God's green Earth is going on with these people?"

Read more: Narcissists use a subtle conversation tactic to make everything about them, and you may not notice it until its driving you crazy

While the tale cannot be verified since the letter is anonymous, Shannon Thomas, a trauma therapist who wrote the book "Healing from Hidden Abuse: A Journey Through the Stages of Recovery from Psychological Abuse," told INSIDER that a story like this could be a case of narcissistic in-laws, which she has seen many times.

"The cruelty shown by in-laws is staggering," she told INSIDER. "It's like narcissism on steroids."

She said the in-laws described in this situation were not only being difficult but purposefully creating a division within the family.

"Their actions are not centered around a strange fondness of mushrooms but a thought-out choice to send a clear and hateful message to their daughter-in-law," she said. "The actions of these parents clearly falls within the psychological abuse spectrum we see from narcissistic in-laws."

So it's actually not about mushrooms — it's about control. And narcissists need control so much that they favor it over the safety and well-being of another person, like a daughter-in-law.

Thomas said this bordered on diabolical and highlighted a lack of empathy and attachment.

"Narcissistic in-laws put a tremendous amount of pressure on a couple," she said, adding that the tension often gets to be so much that the targeted person gives up and leaves their relationship.

"This is especially a risk if the adult child of the toxic parents does not recognize the games being played," she said.

Read more: Here's how victims of narcissists can use the 'gray rock' method when no contact isn't an option

In the letter, the woman said her husband had been supportive. But Thomas said they might still need couples' counseling to help work through any decisions related to his parents' actions and attitudes. For example, maybe they should meet only at restaurants where she can make safe food choices herself.

"Boundaries will be needed, and it would be reasonable for her to refuse to attend any family meals prepared by the parents," Thomas said. "Her safety is the very first priority, and her in-laws have clearly put her in harm's way on too many occasions."