The Mahtmasi, also known as the Carrion Children, are an unpublished additional Vampire Counts Bloodline (type of vampires related by a lineage of being turned leading back to the very first vampires) for Warhammer Fantasy, making them non-canon but still technically part of the IP. Their Bloodline progenitor is canon by virtue of being in a Black Library novel however, making the Mahtmasi Bloodline he created more of a scrap of continuity than a creator fanfiction.
Created for the Night's Dark Masters supplement for Warhammer Fantasy Roleplay, later released by Steve Darlington (the co-writer with Jody MacGregor) on its own.
When Queen Neferata perfected the eternal life experiments of Nagash by inventing Vampirism using her Elixer, she invited her most trusted servants of the court to join her. Among those was her Vizier and Grand Chancellor Maatmeses. Despite his job being to maintain her household and treasury, he instead spent his time indulging himself in earthly pleasures and even as a Vampire spent his time screwing every woman he could. He refused to kill any women as victims, or harm them in any way. He spent Neferata's money like it was his own and bedded every servant he could. Eventually Neferata's husband Vashanesh caught wind of what he was doing, and the fat Vampire chose to immediately flee rather than fight or try to explain himself. Knowing she had spies everywhere and her army was on his heels, he fled into the same Great Desert where Nagash had been stripped to the bone. He ran as long as he could in a state of fear, losing his mind to the paranoia and eventually collapsed on the sand with no strength left in him. The insects of the desert swarmed on his body while the wind and sand dried and tore from him until despite his well-fed flesh repairing itself around the ivnaders he was little more than an emaciated corpse made up more of insect and sand bound to his very being than skin.
He eventually got up and began to travel again, his madness continuing until he found and slaughtered a tribe of Arabyans. He made their greatest warriors into Vampires, unintentionally creating a new Bloodline.
His Vampires continued to spread, consuming/turning entire nomadic tribes while his more intelligent minions carried out assassinations of officials throughout Araby. All accumulate immense wealth, which they keep for their amusement or for bribes to worm their way into another Arabyan city.
Maatmeses, now known as Maht-masa in the Arabyan tongue which means "child of death", lives in a cave in the desert becoming more and more insect as he maintains the order of his Bloodline.
The Strigoi of the south, Mahtmasi have no allies. Everything is an enemy. Unlike Strigoi which have the Strigany to call family, you either are Mahtmasi or not. That being said, Mahtmasi are eager mercenaries and will take payment to not harm a group of beings or even kill their enemies. So long as there is death and food, the Mahtmasi are eager.
The actual goals of Maatmeses himself are unclear. The fluff presents rumors among his children either that he has sworn allegiance to Nurgle or is seeking a way to kill Nurgle and replace him as a Chaos God. Either way he desires the entire world to be Mahtmasi, insects, and mountains of corpses to enjoy.
Mahtmasi Vampires can be considered Ghouls and the other Strigoi options since they reproduce often and feed on meat and blood of everything, dead and living.
They are horrid to look at, being terribly emaciated and having the traits of insects from a chittering language to Bloodline-wide wings. Like their progenitor they have the strange ability to sneak almost anywhere undetected, which greatly helps their assassinations and attacks on settlements. Unique among the Bloodlines is that Mahtmasi have no individualistic streak, and the Vampires live very close as a true family. They are loyal above all to their Chief, who is loyal to Mahtmasi and travels to his cave when called. They celebrate often, gorging themselves and frolicking in corpse piles, feasting on insects like cockroaches and maggots, or dancing covered in viscera and swinging entrails. Those who still feel lust pleasure themselves using dismemebered women, although a strange Bloodline trait that survives from Maatmeses's lifesstyle in Lahmia is some Mahtmasi being unable to harm any pre-menopausal woman or even willingly inconvenience them. The entire Bloodline also inherited Maatmeses's fear of Neferata, and the sight of cats terrifies them instinctually. Since they were never involved with Nagash they didn't suffer the same curse as the other Bloodlines, but instead suffer from a counting obsession, inability to enter someplace uninvited (bribery helps with this one), fire, and allergy to certain metals. They still fear religious symbols as well as sunlight and have no reflection.
Although they turn others into Vampires rarely, they do select the greatest warriors from groups they destroy as well as proud nobility to turn.
They spread themselves throughout lands where humans cannot survive and remain nomadic so no army can be sent to destroy them, feeding on each other to keep their strength up as needed once loyalty and camaraderie give way to survival instinct, while also maintaining secret presences in cities. Only four coastal cities of Araby are yet untouched by them, and in desperation the Sultan has sent for aid from the former enemies in the Old World including Bretonnia to save them.