Once again, in October, Dan Loeb was lobbing grenades. This time his target was Sotheby’s, the international auction house, founded in 1744, that, along with chief rival Christie’s, owns the high-end business of reselling the art, real estate, jewelry, furniture, and other knickknacks of the wealthy. Loeb, the 51-year-old founder and principal owner of the hedge fund Third Point L.L.C., is famous, or rather infamous, for such bomb throwing. Packaging them in the form of letters to corporate C.E.O.’s (and sometimes to his hedge-fund colleagues), Loeb excoriates his targets publicly, not only for their professional performance but also often for their personal behavior. The idea is to humiliate the C.E.O.’s, causing them to quit or to get fired, so Loeb can unleash his strategies for “unlocking shareholder value,” as they say in the hedge-fund world. Other hedge-funders send such letters, but most agree that Loeb’s are the nastiest and most florid.

After Third Point had become the company’s largest shareholder, Loeb sent his Sotheby’s letter on October 2 to William F. Ruprecht, the widely respected chairman and chief executive officer, who has been at the auction house for 33 years. Arguing that Sotheby’s suffered from “a lack of leadership and strategic vision at its highest levels,” Loeb then blasted Ruprecht for his 2012 compensation of $6.3 million, as well as any number of perks—memberships in “elite country clubs,” car allowances, and tax-preparation services—that invoked “the long-gone era of imperial CEOs.” He also relayed the “story” of an “extravagant lunch and dinner” at Blue Hill restaurant, in Manhattan, where, he said, Sotheby’s management “feasted on organic delicacies and imbibed vintage wines at a cost to shareholders of multiple hundreds of thousands of dollars.” Loeb demanded Ruprecht’s resignation and wrote that he would like to join the board “immediately” to begin a search for a new C.E.O. Incredibly, he wrote that he had already identified two internal candidates who could succeed Ruprecht and had begun “informal discussions” with outside candidates as well.

In response, Sotheby’s cited Loeb’s “incendiary and baseless comments.” An unnamed source, speaking to The Wall Street Journal, said Loeb had exaggerated the size of the restaurant tab, which had covered 50 of the firm’s top producers. The company also adopted a so-called “poison pill,” forcing anyone who acquires 10 percent or more of the company’s stock to negotiate with the board of directors to buy the company—a path the activist Loeb is unlikely to pursue. (Ruprecht declined a request for an interview.)

For most of his career Loeb has gone after obscure smaller companies, but the $14 billion war chest he has assembled of his and other people’s money in the last few years has inspired him to pursue bigger game—among them Sony, Yahoo, Morgan Stanley, Apple, Disney—and has attracted unwanted attention to his scorched-earth tactics. Last August, George Clooney, one of Hollywood’s smartest and most genial figures, went ballistic on Loeb, who had bought more than a billion dollars’ worth of Sony stock, and then followed it up with a pair of letters to Sony C.E.O. Kazuo Hirai, claiming that the company’s entertainment division lacked the “discipline and accountability that exist at many of its competitors.” Loeb concluded by assuring Hirai that he “would gladly accept a seat on Sony’s Board of Directors.”

Clooney, after pointing out that he has no particular love for Hollywood executives, rallied to their defense. “I’ve been reading a lot about Daniel Loeb, a hedge fund guy who describes himself as an activist but who knows nothing about our business, and he is looking to take scalps at Sony,” Clooney vented in August to the entertainment-news Web site Deadline Hollywood. “It makes me crazy. . . . A guy from a hedge fund entity is the single least qualified person to be making these kinds of judgments, and he is dangerous to our industry. . . . What he’s doing is scaring studios and pushing them to make decisions from a place of fear. . . . To have this guy portraying it that Sony management is the bad stepchild and doesn’t know what it is doing and he’s going to fix it? That is like Walmart saying, let me fix your town, putting in their store, strangling all the small shops and getting everyone who worked in them to work for minimum wage with no health insurance.”