Mariah Carey says she was betrayed in a huge way by the former personal assistant she hired ... an assistant Mariah says secretly recorded her doing "embarrassing" things and then threatened to release the vids unless the singer coughed up $8 million ... TMZ has learned.

TMZ has obtained a copy of a lawsuit Mariah just filed against Lianna Azarian. 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.

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