Who they were
Leonidas was a king of Sparta of the Agiad line, reigning from about 489 to 480 BC; his birth date is unknown. He died at Thermopylae in August or September 480 BC. Almost everything known about him comes from Herodotus, writing decades later — under the legend, the historical Leonidas is thin.
What they did
He was chosen to lead the holding force of the Greek alliance. According to tradition he took only 300 Spartans — men with living sons — knowing the mission was likely fatal, and the Delphic oracle that Sparta must lose a king or be destroyed is part of the same legend. At the end, with the position betrayed and outflanked, he stayed with his rearguard. The sources give his death and the fight over his body a Homeric color that owes much to their own shaping.
Legacy
His remains were later returned to Sparta, according to Pausanias, and a hero cult honored him there. He became the embodiment of Spartan discipline and self-sacrifice, though the laconic replies attributed to him — such as telling the enemy to come and take the weapons themselves — belong to the legend. Modern memorials stand at Thermopylae and at Sparta, and his name remains a global byword for the last stand.