In terms of the larger worldview, what exactly took place at the Fall of Numenor?
The world was changed from a flat medieval world to the round world of today. Middle-earth was meant to be our own world (see FAQ, Tolkien, 6), and Tolkien’s overall conception was of a progression, with “Mythological Time” changing into “Historical Time”. The events accompanying the Fall of Numenor were a major step in the process. Originally, the “fashion” of Middle-earth was the flat world of the medieval universe. Valinor (the equivalent of Heaven in that the “gods” dwelt there) was physically connected to the rest of the world and could be reached by ship. When Numenor sank (see LFAQ, Humans, 1) “the fashion of the world was changed”: the flat world was bent into a round one, with new lands also being created; and Valinor was removed “from the circles of the World”, and could no longer be reached by ordinary physical means. The Elves alone were still allowed to make a one-way journey to Valinor along “the Straight Road”. (An elven ship on such a journey would grow smaller and smal
