>>86992077A regiment of the Imperial Guard, who's homeworld Cadia was basically the chokepoint for an interdimensional breach in reality.
They held the planet against 12 massive incursions by Chaos, called Black Crusades.
During the events of the 13th Black Crusade, they continued to hold back the forces of chaos, until the leader of the Crusade decided to just blow up the entire planet instead of take it.
Cadians, then, are who people refer to when they say "The planet broke before the Guard did"