First of all, some canon stuff that makes this crossover possible:
-We know that the World is inside a crystal sphere, as the existence of the crystal sphere's surface has been confirmed by star pact warlocks. (DR393)
-We know the existence of some comets and "stars" within the sphere. The difference between "stars" and stars, is that "stars" are actually Elder Evils from the Far Realm. The proper stars are embedded in the shell of the Sphere, although the true nature of the stars remains a mystery for Nerathi sages, with many theories about them: some claim the stars are holes in the shell that surrounds the world, others insist the lights represent the eyes of the gods and other entities keeping watch upon their creation, or the souls of the dead cursed to behold the living at night; while others believe stars are gemstones of impossible size and brightness (the last ones are considered gullible folk). (DR393)
CELESTIAL BODIES OF NERATHSPACE:
- The Sun
- The World
- The Moon (also known as the Goddess by some folk, in reference to Sehanine*)
- Iltani (the second moon, blue-colored; destroyed) (DR382)
- Allabar, the Opener of the Way (a wandering, self-aware planet infested by the Far Realm) (MM3)
- Far Wanderer: A comet that traverses the sky every few centuries and its believed to be related to the gods of good. (AV2-2)
- Hrothmar: A gigantic, wandering comet, actually a sealed primordial. (AV2-1)
- Nebiru: The lair of an apocalypse dragon (a very old and powerful catastrophic dragon) named Nebiru. This lair is composed of half-formed worlds and the rubble of creation. (DR382)
- The Forgotten Sanctuary: A continent-sized landmass, is what remains of a world destroyed by the primordials during the Dawn War. Its inhabited by a few civilizations. (SotAS) [See notes]
- Ulban, The Messenger: A comet of blue-white fire and the morning glory. Its blue-white light disrupts cognition and the ability to recognize danger. Its a being from outside time and space who claims to be the last survivor of the final destruction of the universe. This being seeks for heroes to help him avert the demise of the universe and also reveals insights about the future. Many star pact warlocks are drawn to make pacts with it. (DR366, DU174)
It seems that some stars have come under the influence of Far Realm entities. Such creatures use stars (some without conscious thought, but others with intention aforethought) as windows on the world. It seems unlikely that every star in the sky is associated with a mind-shattering monster (though some star pact warlocks wake from nightmares where every light in the night sky simultaneously blinks open, revealing eyes...). But few stars are definitely so associated, and these connections are scribed in a spidery, shaking hand on a parchment titled Revelations of Melech. There is also a embedded message in the scroll (a sort of psychic engram) called the Confession of Melech. (DR366, DR403)
- Acamar: A corpse star whose motions and behemoth size send celestial objects that draw too close spiraling to their doom.
Caiphon: The Dream Whisperer, a purple star usually on the horizon. It has the guise of a helpful guide star, but sometimes betrays those who rely upon it.
Delban: An ice-white star often visible only during winter, it might surprise the star-gazer with an impromptu flare during any season.
Gibbeth: The Endless. Better not to write or think overlong on this greenish point in the sky.
Hadar: The Ebon Hunger, the extinguished cinder of a star lurking within the cloaking nebula of Ihbar.
Ihbar: The Hands of Tendrils, a dark nebula between stars, is slowly expanding and eating the light of neighboring constellations.
Khirad: The Burning Flame, a piercing blue star, its radiance sometimes reveals secrets and gruesome insights.
Nihal: The Red Worms, a reddish star that writhes around the position it should hold in the heavens.
Thuban: Of the frozen emerald seas. (PHB)
Zhudun: Dead, and of the Blank Face. Another corpse star, historically described as shining a baleful light over the Ruined Realm of Cendriane in the Feywild before its fall.
The "stars" (entities) of the Elder Constellation are not all associated with the Far Realm as other "stars" in the sky are. However, those unfathomable "stars" are inconceivably powerful beings who will survive to time’s end. (DR403)
- Atropus, the World Born Dead: A vast primordial of undeath, spawner of the atropals. Currently, one of the few free primordials. Atropus drifts through the gulfs of space, searching for worlds to consume. When it finds a world, it erases all life from it with a single gruesome touch. (OP)
Father Llymic: A mote of alien thought given form and flesh, who dwells in an icy prison awaiting a time when the world will be right for his arrival.
Pandorym: A god-slaying weapon that personifies the emotionless void of utter annihilation, sealed in the voids of space and awaiting the time when it will be reunited with its monstrous body.
Ragnorra, Mother of Monsters: A primeval source of corrupted life and horrific monsters.
Kyuss, The Worm That Walks: A sentient tear in reality through which one can see a swarming mass of maggots and worms that goes on forever. It's said the "star" Nihal is brother to the Worm, or rather, they are aspects of the same entity as seen from different ends of time.
- Celestian: A notable arcanist and astronomer, he crafted the magic items known as the "Offerings to Celestian" using fragments of the Far Wanderer comet.
- Melech: A mysterious warlock who studied the "stars" and wrote all its discoveries in the scroll known as the Revelations of Melech.
- Tetherya Acrid: A tiefling sorceress fascinated by celestial objects who lived 100 years ago. She founded the Moon Catchers guild to recover all the lost fragments of Iltani.
- Thulzar: A human warlock who founded the Order of the Endless Night and its fortress of Starhaunt 1000 years ago. This order is composed of like-minded warlocks and astronomers who have pledged to protect the World from the evil stars and their spawn. Thulzar mysteriously disappeared a few years after the founding of the order (DU207, MM3)
- Ilshaeren: An elderly eladrin warlock, he is the current leader of the Order of the Endless Night. He is also pledged to the evil star Gibbeth. (DU207)
- Vaden: A brazen human warlock, he is a member of the Order of the Endless Night and is pledged to the evil star Acamar. (DU207)
- Linza: A tiefling warlock member of the Order of the Endless Night, she is pledged to the evil star Hadar. Her current goal is to overthrow her leader, Ilshaeren. (DU207)
The Forgotten Sanctuary is said to be in the Astral Sea, but since in 4e there was no proper distinction between "space" and "Astral Sea" (at least, conceptually), I prefer this continent to be part of Nerathspace, as it is what remains of one of its destroyed worlds. Also of notice is the fact that Secrets of the Astral Sea states that "the Forgotten Sanctuary lies far from the current dominions, and is so out of the way that even githyanki pirates rarely venture there", which fits perfectly with my approach.
Unsourced = 4e Dungeon's Master Guide (p. 160)
* = Heroes of the Feywild (p. 86)
AV2-1 = Adventurer's Vault 2, "Hrothmar’s Gauntlets" (p.59)
AV2-2 = Adventurer's Vault 2, "Offerings to Celestian" (p.117)
DR366 = "Wish Upon a Star" (Dragon 366)
DR382 = Guilds and Groups: "Moon Catchers" (Dragon 382)
DR393 = Class Acts: "Star Crossed: Star Pact Hexblades" (Dragon 393)
DR403 = Class Acts: "Warlock: Strange Constellations" (Dragon 403)
DU174 = "Sealing the Pact" (Dungeon 174)
DU203 = Starhaunt (adventure) (Dungeon 203)
MM3-1 = Monster Manual 3, "Starspawn" (p.187)
OP = Open Grave - Secrets of the Undead (p.16)
PHB = Player's Handbook 1, Warlock's powers list (p.137)
SotAS = The Plane Above: Secrets of the Astral Sea (p.125)