Theros

Theros was a plane that in many ways resembled the mythic landscape and figures of Olympus and a collection of city-states on a Prime Material world that the Olympian pantheon had worshipers on.

Cosmography
This plane consisted of three distinct demiplanes&mdash;the mortal world, Nyx, and the Underworld&mdash;that were all tucked away by its resident deities into their own pocket of the multiverse, away from most of the cosmos.

Calendar
The major calendar system of Theros, the Meletian calendar, marked each day with the beginning of a new moon. Under this calendar, the people of Theros separated each year into twelve months that alternated between consisting of twenty-nine to thirty days, though once every three years the calendar would include a thirteen month.

History
History was a complex thing on Theros as, compared to other worlds, history became a matter of myth far more quickly, with details distilling into generalities, vagueness, and morals. It was often impossible for mortals to distinguish between the historical facts and myths surrounding the activities of their deities on the mortal world.

In the late 1480s DR, a channeling of power from across the planes occurred in the Icewind Dale region of Toril, potentially due to the Everlasting Rime or some other meddling on Auril's part. This meddling with planar magic caused some creatures from other worlds to be dragged into Icewind Dale, including a leonin known as Yanna.

Inhabitants
Some multiversal creatures that could be found on this plane included amphisbaena, basilisks (but only with four legs), catoblepas, cyclopes, blue and red dragons, medusae, night hags, sphinxes, unicorns, harpies, and hippocampi. There also existed satyrs on this plane that, unlike other satyrs found throughout the multiverse, were a species that exhibited both males and females.

One of the uniquely distinct creatures of this plane were leonins, a race of lion-like humanoids that inhabited a golden plain.

Much like on the plane of Olympus, this world was noted for having adventurers who went above and beyond to become legendary heroes, risking everything for glory and not fading into the background.

Trivia

 * Elminster was one of the few (perhaps even only) Faerûnians to be aware of Theros's existence and knew a great deal about the plane.