In Warhammer, Ulgu is the Grey Wind of Magic that is the Aethyr's reflection of the experience of being lost and confused, and was used to power the illusions of the Lore of Shadows. In Age of Sigmar, it coalesced into the Realm of Ulgu.
Ulgu is the Mortal Realm we as an audience know the least about. You can argue this is very fitting, considering its very nature is based on secrecy and illusion.
Ulgu is said to be divided into Thirteen Dominions. We know little about these dominions, except that ultimately all answer to Ulgu's supreme leader, Malerion. The nations of Ulgu are said to rely heavily on political intrigue, backstabbing and assassination. Indeed, many of them had ceased performing conventional war altogether, which came back to bite them in the ass when the Age of Chaos happened and they had no standing armies to defend themselves from hordes of daemons and Slaves to Darkness.
Ulgu is in a mutual orbit with its opposite realm, Hysh. Whenever Ulgu eclipses Hysh, night is caused in the other Mortal Realms. There's also a secret demiplane linking the two realms that partakes of both their fundamental natures at once, called Uhl-Gysh; the Aelfen gods have agreed to keeps its existence their little secret, since this is where they've locked up Slaanesh.
About as little is known about Ulgu before the arrival of the Pantheon of Order as any other realm, but it is notable that several archaeologists based on ancient artifacts believe that the Daemon Prince Be'lakor was the original ruler of this realm before being kicked out by Malerion.
The god of Ulgu, Malerion, and his mother Morathi both woke up in Ulgu during the Age of Myth. After years of wandering, they finally discovered one another by chance. Although both parties had bitter grudges from the World-That-Was (and Morathi was jealous of her son's godhood), they ultimately stuck around each-other primarily out of loneliness, as they were the only other Aelves they had seen in generations. While Morathi wanted to evenly split rulership of Ulgu, Malerion only gave her the Umbral Veil, the shittiest land in Ulgu from which no one but him had had returned, out of spite. He had underestimated his mother, for she managed to successfully tame this deathtrap of a land and created the city of Hagg Nar. Malerion in turn created his own capital city, Druchiroth.
Eventually Morathi and Malerion would team up with their Hysh counterparts, Tyrion and Teclis, to imprison Slaanesh and free the souls inside him to create new aelven races. Morathi would create the Scathborn, creatures resembling hybrids between aelves and gorgons or harpies, while Malerion would create his own race of shadow aelves that we haven't seen but will probably be their own army one day. During this time, Morathi would also co-opt the cult of the dead god Khaine that was growing in her lands, creating the Daughters of Khaine as her own theocratic empire/secret scheme for godhood.
When the Age of Chaos hit, many in Ulgu would flee to Azyr, forming the Ordo Serpentis and Darkling Covens. Malerion and Morathi's forces didn't pussy out, instead beginning a centuries-long guerilla war called the Cathtrar Dule to secure control of Ulgu. This war still goes on long into the Age of Sigmar, and has intensified ever since Morathi's recklessness in taking too many Aelven souls caused the Godseekers of Slaanesh to start thinking Ulgu smells like their missing master.
Other Noteworthy Factions
Aside from the forces of Morathi and Malerion, there are other groups that have set up shop in the Realm of Shadow.
- The Badsnatchers: A strangely brazen horde of grots that regularly pillage the surface world as they believe that the Bad Moon has already covered the whole realm in the Everdank. Though likely false, anyone who tries to tell them other wise will usually find a poison covered shiv in the back soon after. They are led by Ogwotz Da Magnificent, who controls seven large lurklairs in the Hushed Hills as his own kingdom.
- Underguts Mawtribe: Cavernous Ogors with sickly pale skin and a fondness for things that go boom. They live in Mount Bellows, a former duardin hold that the Ogors captured on account of the obscene number of cannons the fortress contained. The Underguts Mawtribe have developed a knack for siege warfare, favoring to tunnel underneath enemy walls and then blast upwards into the back lines with explosive force. Their current Overtyrant is one Grumlog Blisterhands.
- Caengan Lodge: Fiery duardin mercenaries who learned the tricksy and deceptive ways of Ulgu from an ancient nation called the Shroudling Kings. Said ways were turned against the Shroudlings when they refused to pay the Lodge for their services. The Caengan Lodge razed the kingdom and took their payment and retribution as recompense.
- Clans Eshin: The assassins and spies of Skavendom are mostly concentrated in the Realm of Shadows, though (big surprise) no one knows where exactly they’re hidden.
- Misthåvn: The “City of Scoundrels” was created from massive ships of varying races all lashed together. It’s infamous among the Azyrites for its corrupt policies, underhanded combat strategies, and a very lucrative illegal narcotics market.
- Godseekers of Slaanesh: Morathi’s aforementioned meddling with souls has drawn the attention of the Dark Prince’s most devout followers who scour them Realms for any clue of their master’s whereabouts about how to release him. Their most recent engagement was assaulting the Umbral Veil whilst Morathi enacted her scheme to become a goddess.
|The Nine Realms of the Age of Sigmar|
|Aqshy||Realm of Chaos||Ghyran|