Fearsome cartel kingpin Joaquin "El Chapo" Guzman is just starting his life sentence — but a new, even more terrifying drug lord is already taking his place.

Nemesio Ruben Oseguera-Cervantes — nicknamed El Mencho — is the boss behind Mexico's Cartel Jalisco Nueva Generación, or CJNG.

And experts believe it's fast becoming the country's deadliest — and possibly richest — cartel.

Born into a poor, avocado-growing family in the Mexican state of Michoacán in 1966, El Mencho left school at age 10 to help out on the farm before taking a job guarding a marijuana crop at 14 before crossing the border into California to deal drugs.

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But in the early 1990s, he and a cousin were busted trafficking and were deported back to Mexico — however, he kept his crimes a secret and was able to score a job as a police officer in the state of Jalisco.

Nemesio Oseguera-Cervantes - aka El Mencho - has a $US10 million reward on his head. Photo / DEA
Nemesio Oseguera-Cervantes - aka El Mencho - has a $US10 million reward on his head. Photo / DEA

He eventually left the force to work for the Millennium Cartel, which was allied with Joaquin "El Chapo" Guzman's own gang, the Sinaloa Cartel — but around 10 years ago, he set up his own criminal crew which quickly gained a reputation for especially brutal mass killings.

These days, Oseguera-Cervantes, 52, is widely believed to be a billionaire — and he's got a $US10 million bounty on his head thanks to his position at the top of America's Drugs Enforcement Administration's (DEA) most-wanted list.

His cartel dealt crystal meth, cocaine and heroin, with the drugs ending up across the world — including here in Australia.

In the words of Paul Craine, who fronted the federal DEA in Mexico during El Chapo's 2016 arrest, El Mencho is "public enemy number one" with an army of "thousands of bad guys".

Narcos expert Daniel Solis told UK tabloid The Mirror, Oseguera-Cervantes should be feared.

"He runs his cartel like a paramilitary," Solis said. "Its arsenal, and its organisation put the regular army to shame. With the insight he gained while in the police, Mencho knows the power of a well-structured force.

"He has publicly stated he will die ­fighting rather than be taken alive. He expects the same of all his men."

Even more frightening was Solis' comparison between CJNJ and Islamic State.

Joaquin 'El Chapo' Guzman was convicted this month of running an industrial-scale smuggling operation after a three-month trial. Photo / AP
Joaquin 'El Chapo' Guzman was convicted this month of running an industrial-scale smuggling operation after a three-month trial. Photo / AP

"Its method of ­killing is more akin to ISIS than the ­cartels," he said.

"Never ­before has how they kill, in the numbers that they kill, been seen in Mexico."

And in an interview with Rolling Stone, an unnamed former DEA agent said El Mencho's cartel had "way more money than Sinaloa".

"Mencho has been very, very aggressive — and so far, unfortunately, it's paid off," another anonymous investigator told the publication.

Over the years, El Mencho's notorious brutality has seem him become one of Mexico's most feared criminals — including his debut on the cartel scene, which involved the dumping of 35 mutilated bodies in the major port city of Veracruz during peak hour.

kidnapping the sons of a millionaire rival, and ordering the rape and murder of a 10-year-old girl mistakenly thought to be the daughter of a different rival.

And in 2015, his henchmen killed a man and his young son by strapping dynamite to their bodies before setting it off.

That same year, eight soldiers and a police officer died during a failed raid which had tried to take down the drug lord.

In retaliation, El Mencho's men set fire to scores of vehicles and buildings, devastating the state with 10,000 troops needed to bring the situation under control.

With El Chapo behind bars following a lengthy, sensational trial, El Mencho is now firmly in the sights of the authorities — but few believe the crime leader will ever be taken alive.