Master Players' Book - Featuring rules for new weapons, weapon masteries, and sieges. Also includes more advanced rules and spells for your master characters. Master DM's Book - New rules for anti-magic effects, mystics, artifacts, and campaigning. Plus new treasures and monsters, including creatures from other planes.
The levels of power for characters just seem to go up and up. Hack and slash on the grandest level possible. Reviewing this work however brought me to remember the deadliest monster of all time. The undead beholder. Fear the horror and cry to the vampires who give comfort, hugs, and kisses compared to the monster who can even best dragons with little effort.
I had seen this book when I was maybe 13 years old and I only skimmed through it then.
In a 5e world, seeing D&D's progression to 36th level is just fun. Negative AC seems wrong now at days. The Lich in the monster section was memorable from when I read it all those years ago. This is the book I would have liked to have had in my collection. Today they are so expensive, I prefer the DrivethruRPG pdf that is available.