TMZ Sports has obtained the complaint in which Cube and Jeff Kwatinetz are suing several individuals including Sheikh Abdullah bin Mohammed bin Saud Al Thani ... the CEO of the Qatari Investment Authority.

Cube says the Qataris were supposed to invest $5 MILLION and be passive investors in the league -- but instead "quickly started to insinuate themselves into the affairs of BIG3 despite failing to live up to even their most basic obligation to fully fund their investment."

Cube says the Qataris would openly brag about their individual wealth -- with one guy, Ahmed Al-Rumaihi, even blowing $700k during a gambling session in Vegas following the BIG3 championship game.

"These members and associates of the royal family made excuse after excuse for not paying, all of which is documented in text messages and emails, where the blame for their failure to fund the millions they owed the BIG3 ran the gamut from their 'sinuses,' 'hiking,' it being a 'long day bro,' and to bad press regarding Qatar associations with alleged funding of terrorism."

"Also, like a simple debtor in hiding from a collection agency, these Defendants with their purported links to the Qatar royal family, would go into hiding and refused to return phone calls and ignore Plaintiffs."

Cube says in Dec. 2017, Kwatinetz attempted to set up a meeting with Al-Rumaihi who instead of substantively responding "sent a picture of his teeth being worked on, apparently to conjure sympathy."

, Cube claims Al-Rumaihi eventually got so pissed off over the repeated requests for the money that he loudly screamed at Kwatinetz and threatened his life saying, "You don't know who I know in L.A. and what they're capable of. You should think of your safety and the safety of you and your family."

Cube and Kwatinetz are now seeking $1.2 BILLION in damages in order to pay $20 million per player in the league.