The Mandalorian Supremacists were a group of Mandalorians who considered all non-Mandalorians inferior. During Pol Temm's tenure as eleventh administrator of The Wheel, they attacked the station and destroyed Temm's Defender-class Star Destroyer, killing him.
Sources
Viva Space Vegas! The History of the Marvelous Wheel: Part 2 on StarWars.com (original link is obsolete) (First mentioned)