Caldera's complaint, filed in the New York Supreme Court, alleges that Apollo pursued a campaign of "systematic defamation and interference," including "disparaging the company in the marketplace."

Caldera is also asking for at least $1.5 billion in damages, according to the filing.

"We strongly believe Caldera's claims completely lack any merit, and we intend to defend ourselves vigorously," Apollo spokesman Charles Zehren said in an emailed statement.

Caldera and Apollo's life insurance unit Athene Holding Ltd are both bidding for control of American Equity Investment Life Holding Co, a U.S. provider of annuities and life insurance products, according to a Financial Times report on Thursday.

In May, Iowa-based American Equity said it would explore a sale, attracting interest from firms such as Athene Holding and FGL Holdings. Thursday's filing alleges that Apollo hindered Caldera from competing with Athene, causing rifts between the company and its investors.

Caldera could not be reached for comment, outside regular U.S. business hours.

(Reporting by Nivedita Balu with additional reporting by Ismail Shakil in Bengaluru; Editing by Sunil Nair)