Short answers, pulled from the story.
Frank Mentzer wrote the Dungeons and Dragons Immortals Rules. It was published by TSR in 1986 as the fifth and final boxed set in Mentzer's revision of the D&D game system, which he had been building since 1983.
The Immortals Rules covers characters who have transcended the level system entirely. It follows the Master Rules, which supported levels twenty-six through thirty-six, and replaces levels with a rank system based on power points.
When a character achieves immortality, all accumulated experience points are exchanged for power points at a rate of ten thousand experience points to one power point. Players then spend power points to raise attribute scores, purchase new abilities, and maintain their current rank.
The outer planes range from mono-spatial atto-planes roughly one-third of an inch across to penta-spatial tera-planes approximately eight hundred fifty-one billion light-years in size. Both exist within the same cosmological system presented in the set.
Graeme Davis, writing in White Dwarf issue eighty-three, called the set "the culmination of the D&D game system" while admitting he could not imagine actually playing it. Lawrence Schick, in his 1991 book Heroic Worlds, described the rules as so different from low-level D&D as to be almost another game entirely.
The set contains the Player's Guide to Immortals, which runs fifty-two pages, and the DM's Guide to Immortals, which is thirty-two pages. Both were written by Frank Mentzer, edited by Anne Gray McCready, and illustrated by Larry Elmore and Jeff Easley.