Rhadamanthus was, in Greek mythology, a judge of the underworld. This judge's name was used as a spell-casting word by Lucien.
After the USS Enterprise in 2269 traveled to the Megan universe, Lucien appeared on the starship's bridge whereupon he uttered this name. The ship's systems returned to normal and it was in orbit of Megas-Tu. (TAS: "The Magicks of Megas-Tu")