Short answers, pulled from the story.
The Dungeons & Dragons Master Rules arrived in 1985 as the fourth pillar of a five-part expansion designed to take players beyond the limits of mortal existence. This boxed set was released to bridge the gap between powerful heroes and the godlike Immortals that would follow.
Frank Mentzer engineered the Dungeons & Dragons Master Rules expansion to bridge the gap between powerful heroes and the godlike Immortals that would follow. He took the reins of the Dungeons & Dragons franchise in 1983 and wrote the books himself.
The Dungeons & Dragons Master Rules provide a complete system overhaul for characters ranging from level 26 to 36. Before this boxed set, the game had effectively ended at level 25, leaving adventurers with nowhere to go but to stop playing or invent their own house rules.
The Dungeons & Dragons Master Rules introduce a new class called the mystic and a sophisticated system called Weapons Mastery. These mechanics allow characters to rise from the rank of Novice to Grand Master through dedicated practice and experience.
Paul Cockburn reviewed the Dungeons & Dragons Master Rules set in issue 73 of White Dwarf magazine in January 1986. He rated the set 8 out of 10 overall and described it as an intelligent, subtle, and interesting extension to the game.