The True Mandalorians was the name given to the majority of Mandalorians who remained loyal to Jaster Mereel upon him becoming Mandalore and forming the Supercommando Codex.
They were opposed in the Mandalorian Civil War by the Death Watch splinter group. They attempted to return the Mandalorians to their past state of honorable mercenaries, as opposed to the depths of brigandage to which they had sunk in the past centuries.
They were largely destroyed in the Battle of Galidraan. There were only two survivors,these being Jango fett and Silas.