The Legion of Super-Heroes has been on the fringes of the Arrowverse ever since a Legion Flight Ring was cited when The Flash first ran through the Multiverse. That same Flight Ring was later seen in Superman's Fortress of Solitude. Supergirl at last made it official when Mon-El (Chris Wood) returned to the series and revealed he'd spent 7 years in the 31st century and had even married Imra Ardeen (Amy Jackson) from Titan, also known as Saturn Girl.
In Supergirl's fall finale 'Reign', the Legion of Super-Heroes was finally name dropped, making the 31st century superhero squadron officially a part of the Arrowverse and its future. Mon-El and Imra are currently in National City after spending 12,000 years in cryo-sleep before being awakened by the torpedo attack that damaged their ship resting underneath the city in the season 3 premiere. As they try in vain to repair their ship, the 31st century couple are living at DEO headquarters, where they provided a download of information on what life is like in a thousand years.
Related: Who Are The Legion of Super-Heroes?
Mon-El painted a picture of a 31st century filled with constant conflict and strife. The governments of the world were overwhelmed by violence. In response, Mon-El organized the various super powered aliens together into a team to help Earth's leaders maintain peace both on Earth and in outer space. They dubbed themselves "the Legion." As inspiration, Mon-El based the Legion's principles on Supergirl (Melissa Benoist) and her heroic example.
Like the Girl of Steel, the Legion is committed to a compassionate fight for justice and protecting others. Supergirl's very example gave Imra's life purpose and is what made her join the Legion. According to Mon-El, the Legion still isn't nearly enough to fight 'the darkness' spreading in the future, which is perhaps a tease that Supergirl will adapt The Great Darkness Saga, one of the seminal Legion of Super-Heroes comics stories where the Legionnaires fought Darkseid himself.
As for how Mon-El and Imra arrived in the 21st century, they were on a mission when their ship fell into one of the wormholes that brought Mon-El to the future in the first place. Their ship crashed on Earth 12,000 years ago, so they went into cryo-sleep, planning to awaken in the 31st century. There are other cyro-sleep pods in their ship promising a handful of other Legionnaires still to be revealed, and fans also await Imra to officially go by her comic book moniker Saturn Girl.
Supergirl as the inspiration for the founding of the Legion is the series' spin on their classic comics origin where the young teen heroes of the future modeled their team after the example of Superman before they traveled back in time to meet Superboy. Clark Kent in his boyhood years and later as the Man of Steel would spent time fighting alongside the Legion in the 31st century as well. Smallville season 8 introduced their version of the Legion, who traveled back in time to meet Clark Kent (Tom Welling). Of course, Supergirl has appropriated various aspects of the Superman mythos, and it makes sense for this incarnation of the Legion to be so intimately tied to the Girl of Steel, with Mon-El as one of its founders.
Related: Mon-El's Return Explained
Time will tell if Supergirl will bring in even more members of the Legion like Cosmic Boy, Lightning Lad, Brainiac-5, or even Matter-Eater Lad, but the Arrowverse has proven how adept it is at taking even the silliest DC superhero concepts (Vibe, Elongated Man) and making them work wonders. Kara Danvers traveling to the 31st century is also an exciting possibility, and something fans might be able to look forward to in the very near future.
Supergirl season 3 returns on Monday, January 15th @ 8pm ET on The CW.