For many of their nearly 10 years as romantic partners, Irina Bolgar said, she and Pavel Durov enjoyed a luxurious lifestyle. The extravagance grew as Telegram, the messaging app Mr. Durov created, became a worldwide hit and cemented him as a billionaire and one of the world’s most powerful tech executives.

During the summers, they would spend about $1 million for a month at a 116-acre resort in Sardinia. In Dubai, they stayed in a beachside penthouse with its own elevator. Private planes ferried them to Paris, Italy and Monaco.

But something changed during that period, according to Ms. Bolgar, who is now entangled in a legal dispute with Mr. Durov. She said he morphed from a principled entrepreneur whom she admired into an increasingly arrogant, controlling and finally abusive adversary.

According to a Swiss criminal complaint Ms. Bolgar made last year against Mr. Durov, he abused their youngest son five times in 2021 and 2022. On one occasion, Mr. Durov struck the child in the back, sending him across the room, she said. In another, he shook him so hard the boy’s shoes came off. Later, he grabbed the child by the leg and said he would kill him.

The details are included in public documents related to the criminal case, which was previously reported by Forbes. The files do not list the names of Mr. Durov, Ms. Bolgar and the children, but there are a number of identifying factors, including the case number for a civil child custody complaint made by Ms. Bolgar.

A spokesman for prosecutors in Geneva confirmed that an investigation was underway but declined to comment further. Mr. Durov’s spokesman said in a statement that the incidents “never occurred” and that the accusations “lack substance.”