@tetrasodium
Nice quote!
I am guessing this is the most recent Eberron book, that isnt WotC, but is in DMsGuild?
Thelanis equates to the Feywild. This book emphasizes Thelanis as a realm of fairytales. The book suggests four different kinds of spirits within Thelanis. 1) The archfey who write the stories. 2) The barons who are central characters. 3) The "supporting staff" who magically appear because a story mentions them, and might be anyone from a giant, to a sprite, to a bird, to a tree.
Notably, the Feywild seem to be the only ones of the three who have freewill, and are immortal. The supporting cast lack freewill. While they lack freewill, they are immortal, in the sense of reliving the same story again and again. But if one can pull them out of their story (by encouraging them behave in a way that contrads the story?), they gain free will (thereby become "humanoid" in the new sense of an individual with freewill), thus become a "mortal", who can exit the recurring story.
Note Thelanis resembles the Dreaming well enough, if each story is understood as a separate dream.
The eladrin elves of Thelanis seem to be their own kind of creature, separate from this storytelling hierarchy. The eladrin are "mortals" ... sorta. The text mentions that there are immortal eladrin archfey who live among these eladrin in the feyspire cities. So at least some eladrin are immortals. Moreover, when the eladrin enter the Material Plane, they become "more mortal" (whatever that means), implying that every eladrin elf is "more immortal" while in the Feywild.
With regard to the Level Up playtest, even the Eberron eladrin elves have a lifespan that is often mortal (six hundred years) but some elves continue to live immortally. Eventually, the immortal elves might serve as eladrin archfey.
My personal preferences for the eladrin in Thelanis.
• The Fey elf stories are different from the stories of other archfey. The eladrin stories refer to the fates of individual humans.
• Each story describes a different timeline for a human life. A human might switch from one timeline to an other, to change their fate.
• The Fey elves can also alter each others fates, in the same way.
• The "Moon Court" seems earthy, among trees, more Celtic.
• I would like a "Sun Court" that is skyey, as sunbeams among sun and clouds, more Norse.
• The Sun Court should be a democratic parliament, comprising the members of each elven family, who each vote.
• A jarl (elected prime minister) is called a Songster.
• There is also a Lawspeaker, who keeps track of the previous votes, and serves as a kind of supreme court.
• It seems like a bard-ocracy, but probably any mage can be elected, including druid.
• The stories of the jarl reflect the politics of the parliament. Political intrigues among elven voters, can alter the course of human history.