Bestselling author Walter Isaacson, who observed Musk’s day-to-day life for about two years, was discussing the tech tycoon on a Twitter Spaces interview ahead of the release of his biography on the Twitter, Tesla and SpaceX owner.

But he didn’t take credit for coming up with the term to describe Musk’s alter ego. In fact, he revealed that Musk’s former girlfriend and mother to two of his children, the musical artist Grimes coined “demon mode”.

She allegedly told the writer that when the tech tycoon shifts to such a headspace he can be “unpleasant” to be around, but it’s how he “gets sh-t done,” according to Insider.  

Fortune has reached out to the Canadian music artist, whose given name is Claire Boucher, for comment.

Many leaders tap their beast mode

It can be useful for leaders to shift into beast mode on occasion, such as during a company crisis or when deciding whether layoffs are the only way to effectively cut costs and keep investors happy.

It’s why, according to Isaacson, many of the successful people he’s written about, including Steve Jobs, do exactly that.

By tapping into their “dark streak”,  Isaacson said that leaders were less weighed down with empathy and were able to focus on bigger-picture issues.

But in Musk’s case, Isaacson suggested that his dark streak sometimes actually hinders the business and frightens workers.

This was especially the case when his employees didn’t match his “maniacal sense of emergency”—”He’d go dark and I’d know that he was just going to rip that person apart,” Isaacson said, according to Insider. He added that this happened often when Musk took Twitter’s helm and implemented mass layoffs. His outbursts were at times “uncomfortable” to observe.

“He is just brutal,” Isaacson reportedly added. “The thing that I noticed is that once he finishes doing it—and it was never physical and it was almost done in a flat monotone—but he would just really attack people and then a few days later, if they absorbed the lesson, he’d forget about it. It would be as if he went from becoming Dr. Jekyll to Mr. Hyde and then didn’t even think that much or remember that much of how tough he had been on people.”

Musk explained to the writer that it’s a form of “egotism” to want to be empathetic to one person and jeopardize the bigger picture. But according to Isaacson, his “brutal” approach is only effective about 80% of the time—for the other 20% it’s not only “problematic” but his people are actually “afraid to give him bad news” presumably in fear of his reaction.

This revelation isn’t necessarily new

According to Insider, the biographer’s book, which hits shelves this September, dives into how the world’s wealthiest person’s complex personality balances his “dark streak” with his work at Tesla and SpaceX.

However, readers shouldn’t be too surprised by Isaacson’s insight into Musk’s personality—another biography on the billionaire has already pointed out Musk’s apparent short fuse.  

The Wall Street Journal‘s Tim Higgins laid bare Musk’s habit to unleash the beast on anyone—from hourly workers to strangers to top executives—in Power Play: Tesla, Elon Musk, and the Bet of the Century. 

Meanwhile, Musk’s tough approach to work is hardly a secret. 

The serial entrepreneur has publicly encouraged staff to prove their dedication to their job by sleeping at the office (or under their desk, like he’s done in the past), work a minimum of 40 hours per week in the office and burn the 3 a.m. oil. 

Twitter responded to Fortune’s request for comment with its customary poop emoji.