Tonight's episode of Star Wars Rebels, "Imperial Supercommandos," introduced us to some new Mandalorians, who have signed on with the Galactic Empire. These jetpack weating warriors are led by a man called Gar Saxon. While the show mentioned that he's a member of House Vizsla, same as Sabine, it didn't go too much into his backstory. Indeed, fans of Star Wars comics have already met him!
Gar Saxon has been called a "Supercommando" before - previously, he held that title as a member of Death Watch, a band of Mandalorian extremists who were once led by Pre Vizsla during the Clone Wars (and on Star Wars: The Clone Wars). While he was never introduced on that show, Saxon would have been during a later arc that was developed but never filmed, due to the show's fairly sudden cancelation after Disney purchased Lucasfilm four years ago.
That arc did get released, however, as the comic book series "Darth Maul: Son of Dathomir." in the series, we were introduced to a group of Mandalorians who maintained loyalty to Darth Maul, and Saxon, alongside Rook Kast, was a primary member of that group. He broke Darth Maul out of prison following his capture by Darth Sidious toward the end of The Clone Wars series. They took Maul to the Shadow Collective, the group of various underworld agencies he has assembled together to rival the Republic and Separatists alike, on the planet Zanbar.
There, they were attacked by General Grievous and a droid army, but the leaders, including Saxon and Maul, escaped. They confronted, and defeated Grievous again on Ord Mantell, and even captured Count Dooku, the powerful Sith Lord known as Darth Tyranus. Despite these individual victories, and fighting four Jedi to a standstill, the best they could claim is that they escaped with their lives.
We don't yet know what happened to Gar Saxon in the interim years, from the establishment of the Galactic Empire to his joining of it around 15-16 years later, but as he's worked with the Sith before, and Darth Maul is still out in the galaxy as well, it's clear that Saxon's story isn't over.