"Magastromancy" was the name invented by John Gaule in 1652 for magical astrology; so "magastromancer" was one who practiced magastromancy and "magastromantic" pertained to magastromancy.
The French King Louis XI was a staunch believer in the prophetic powers of astrology, and of one astrologer in particular, who correctly foretold the death of a court woman eight days before it occurred. At the same time, Louis feared supernatural powers, and after vacillating for a while hatched a scheme to have the astrologer thrown to his death from an upstairs palace window.
He lured the unsuspecting man to his chambers and, as the guards were about to give the stargaser the heave-ho to the stone courtyard below, the astrologer quickly extemporised a story that gave the king permanent pause for thought. The astrologer saved his own life by cleverly prognosticating that he himself would die just three days before the king.