Original: He is primarily known as one of the founders of the Hermetic Order of the Golden Dawn, a ceremonial magic order of which offshoots still exist today.
Simplified: Samuel Liddell (or Liddel) "MacGregor" Mathers, born as Samuel Liddell (January 8 or 11, 1854 - November 5 or 20, 1918), was one of the most influential figures in modern Occultism.