Often terrifying to look at, gang members sometimes cover their entire faces in tattoos.
They are known to be particularly deadly, and use violence against people who disrespect them.
More than 100,000 members across the globe have joined the group. Some of them have been convicted of rape or murder, motivated by revenge.
Some members are as young as eight and any new member has to suffer through an initiation of being brutally kicked and punched.
The gang above all values loyalty and if any gang member proves to be disloyal, they will be punished and either murdered or have their hands cut off.
There are many members who now live in El Salvador, and they're so vicious, a separate prison has been set up just for them.
Those in lock-up thrust their hands through metal prison bars and shape them like devils' horns - and even some prison guards are too scared to go to the cells.
El Salvador is one of the most violent countries in the world, according to United Nations, which said it had a 2012 murder rate of 41.2 homicides per 100,000 people, projected to have become 91 per 100,000 in 2015.
GANG ATTRACTED TO AUSTRALIA'S DRUG MARKET
The Australian Federal Police is remaining tight-lipped about the gang but it is believed they are attracted to Australia's booming drug market.
The gang has previously been linked to Mexican drug lord El Chapo and they have plans to team up with Australia's outlaw motorcycle gangs.
US Homeland Security's special agent in charge, Mark Selby, told the Herald Sun it was an extremely violent gang to be entering Australia.
"MS-13 are opportunists ... We are concerned that (if) they partner with criminal groups like outlaw motorcycle gangs, it will make them more devastating," he said.
The gang mainly focuses on extortion and drug distribution and Selby said members "are brutally violent".
The Herald Sun reports the gang could be storming Western Australia, NSW and Victoria as well as South Australia.
Frighteningly, MS-13 wants to be the gang that has the highest number of killings across the world and the gang may have been infiltrating Australia secretly and slowly for some time.
There have been reports the inked up members are looking for "cleanskins" to join their group so they can commit crimes inconspicuously.
They first made their presence in Australia known when they tagged a garage door in Bondi with their gang name in 2015.
Letterboxes and walls in the suburb were also sprayed with bright blue graffiti and residents became fearful.
Australian Criminal Intelligence chief executive Chris Dawson told the Daily Mail Mexican cartel activity had started to emerge in Australia around 2014, which could be a sign the gang is establishing itself in Australia.
"In July (2014) the Australian Crime Commission was involved in the arrest of two men allegedly linked to the drug cartel," he said.
Selby believes Australian authorities will be able to handle the gang because of their experience with the outlaw bikie gangs.
"I've no doubt the authorities will be able to deal with it," he said.
