From Reddit (https://www.reddit.com/r/AoSLore/s/MlDslvb2Tz):
Loremasters: Ushoran reveals (DAWNBRINGERS IV HUGE SPOILERS)
So Warhammer+ has made a video about Ushoran's lore, and it pretty much spoils the entirety of the Ghyran side of the plot for Dawnbringers IV. I'll start by revealing some minor new details that don't really spoil much before going into the full spoilers onwards.
*Ushoran's Mace and Chalice (which may or may not be his original Chalice) were given to him by the Hollowmourne, who presumably found them in their deluded quest for artifacts.
*Ushoran's castle in New Summercourt is too advanced to have been created by Ghouls, and in fact was created by a race of Gargants who were kin to the Sky Titans of the Scabrous Sprawl.
*Fang of Nulahmia is a term that applies to Neferata's most elite agents, so Sekhar isn't the only one.
*Ouburoth the Time-Swallower was the God of an Underworld called "Kandoor" (don't know the proper spelling). Its worshippers would willingly allow it to eat their souls, believing that would ensure their reincarnation. Ouburoth was so powerful that even after being weakened by Sekhar undermining its cult, Nagash couldn't devour him entirely, resulting in him spitting out the snake you see on the model. Nagash orders Sekhar to use her hypnotic abilities to keep Ouburoth under control and prevent it from regaining its full power.
*The ships exporting Kingsblood are captained by Neferata's vampires but crewed by ghouls at Ushoran's insistence.
Okay, now onto the actual story.
>!The Ghyran Crusade, after exiling a bunch of its members who formed a cult called the "Kin of the Stag" inspired by Belthanos, end up at an Ossiarch Bonereapers fortress. The fortress was weakened by Alarielle's Rite of Life, so between the Ironweld artillery, Dispossed clans and the life magic of Iscilla Thorian, the Dawners are able to break down the fortress and move into New Summercourt.!<
>!Once there, the Crusade meets up with Astreia Solbright and her Stormcast, who are looking for Ushoran because they believe he knows a way to heal their souls. A lot of the people in the Crusade want to go to the biggest castle on the biggest mountain because they think the "Sombre Paladin" can help them, though they do not know where they heard that name. As such, an elite force of veteran Dawners (including Thorian) alongside Astreia go to investigate the castle.!<
>!The castle's gate opens, revealing Sekhar surrounded by a horde of ghouls, who welcome them as guests of the Summerking. The horrified Dawners are brought into the castle and seated at a feast where they meet Ushoran. Such is the aura of delusion coming from Ushoran that all the mortals see him as a magnificent king, and it even takes all of Astreia's willpower to see him in his true form. Ushoran is a polite and courteous host, even though the Dawnbringers are essentially his prisoners now, and is eager to hear them tell stories of the world beyond. When Solbright tells him of her quest to find him and seek a cure for the Flaw, he seems genuinely moved by her efforts.!<
>!Sekhar uses her hypnotism to make one of the Dawners shoot Ushoran, a plan she cooked up with Grand Justice Gormayne, who is her co-conspirator in his lucid moments. Ushoran retreats and the ghouls enter a frenzy; however, instead of attacking the Dawners, they're attacking the vampires, with even Gormayne falling into madness and joining them. Ushoran had even orchestrated the seating arrangements for the feast so the vampires would be surrounded and trapped by the ghouls, and Sekhar and a handful of her vampires are barely able to cut their way out.!<
>!Astreia pursues Ushoran, and when she finds him he's acting lucid. He thanks her for being an audience to his lunacy and helping him remove the Nulahmians, who are no longer useful to him. He doesn't see a point in indulging her curiosity any further, and savagely kills her.!<
>!The ghoul crews of the ships that export Kingsblood, upon hearing of Neferata's treachery, mutiny, tying their captains to the wheel and forcing them to sail. The Kingsblood distribution network is now fully under the control of the ghoul cults.!<
>!It ends with Neferata appearing to Ushoran as a similacrum made of blood and telling him that if he is going to repeat his game of rebellion against Nagash, he will meet the same fate he previously did. Ushoran isn't afraid though; he's prepared a long time for this moment, and now is the time he asserts himself among the hierarchy of Death. No real mention in the video of how the Ghyran Crusaders get out alive, if they do.!<