TMZ has obtained a copy of a lawsuit Mariah just filed against Lianna Azarian, also known as Lianna Shakhnazarian. Mariah says she hired Azarian back in March 2015 as an Executive Assistant. Mariah claims it wasn't long before Azarian went to town with her credit card, buying personal stuff and also claiming to retailers she was buying things for Mariah, trading the singer's goodwill to get deep discounts.

Carey says unbeknownst to her, Azarian was secretly filming her "personal activities" ... which would be "embarrassing," and extremely damaging both personally and professionally if revealed.

Mariah says Azarian showed some of the videos to friends, saying if she was ever fired she'd sell them.

Carey claims she was paying Azarian as much as $327,000 a year, but ended up firing her in November 2017, and that's when she claims the blackmail began. Carey says Azarian demanded $8 million and if she didn't get it she'd release videos and other damaging information.

The lawsuit does not say what's on the videos ... although some are described as "intimate."

Mariah says she's demanded the videos and other evidence of criminal activity back, to no avail. She's suing for unspecified damages, but wants more than $3 million.

9:34 AM PT -- A source close to Mariah tells TMZ, "This new year welcomes Mariah's continued efforts to clean the trash from her life."

They continue, "According to a Complaint filed today in California, an executive assistant employed in 2015 to help with business and personal matters, turned out to be a grifter and extortionist. Because her threats and bad acts are too great to be ignored, Mariah has been compelled to file a lawsuit against her."

"Given that the evidence against this former assistant is vast and deplorable, we anticipate a victorious resolution. Mariah continues her streak of success this year with an upcoming North American tour and return to Vegas."

Originally Published -- 3:38 AM PT