The Mystaran Immortals are the entities that fulfill the role of gods in the Dungeons & Dragons setting of Mystara. Unlike "normal" D&D deities, all Immortals were once mortal beings, but managed to uplift themselves to a god-like status through one of four Paths that require tying themselves to one of five Spheres. This typically requires the aid of an existing Immortal to act as a Patron.
The Four Paths to Immortality are:
The Dynast: Somebody who erects a great and long-lasting empire.
The Hero: Somebody who achieves the ultimate ideals of heroism.
The Paragon Somebody who reaches the ultimate in their chosen profession.
The Polymath: Somebody who dedicates themselves to learning, to the point of living multiple lifetimes in multiple different roles.
Certain Paths are modified by aspiring immortals who belong to unique races. For example, would-be Therianthrope Immortals, as covered in Creature Crucible, have to specifically direct their Path along a route relating to their shapechanging; the Therianthropic Dynast must found an empire of at least 10,000 therianthropes; the Therianthropic Hero must "pass on" their mantle to a successor of the same therianthropic strain; the Therianthropic Paragon must teach multiple shapechanging mage apprentices how to master their beast-man forms, and must be acknowledged as the alpha werebeast by all other werebeasts within 1000 miles; finally, the Therianthropic Polymath must contract a different form of therianthropy in each of its different lives.
Five Spheres, representing the five foundational elements of reality itself, serve as the ultimate power behind the Mystaran Immortals. These Spheres are Matter, Energy, Time, Thought, and Entropy.
In practice, Entropy is treated as the "Evil Sphere" and PCs aren't allowed to become Entropic Immortals, who basically fill the roles of Demon Princes and Archdevils in the other versions of the D&D multiverse.
Mystaran fey claim that, in a distant age, Matter/Energy/Time/Thought were actually a singular Sphere called "Life" whilst Entropy was the one broken into four components; the fey are remnants of that cosmic era, and are confident that one day the change will reverse and the Spheres will shift in nature once again.