The sibriex is an obyrith, that is, a demon of the Abyss, but not a tanar’ri, as most demons today are. Obyriths are a much older form of chaotic evil, almost entirely gone after the tanar’ri took over the Abyss and wiped them out at the end of the War of Law and Chaos. As such, a sibriex is far older than almost all demons you will meet, and much more powerful than most.
They were originally introduced in Fiendish Codex I: From the Abyss for D&D 3.5e. There, they had an innate affinity for fiendish flesh-grafting, and could apply grafts to creatures freely and instantly, which was a significant power. For their own uses, this was mostly used to experiment and torture creatures, but they could and did use it to assemble and upgrade their minions. They could also be bound by mortals, using planar binding spells, who wished to take advantage of this ability and gain fiendish grafts of their own, but doing so was fraught with peril as the sibriex was not easily bound.
The next time the sibriex reappeared was in Demonomicon, for D&D 4e. Now the sibriexes were divided into multiple kinds, such as the chain master and the flesh crafter. D&D 4e being what it was, most of the details on the flesh crafter were about how it fought in combat, its use of flesh-warping disease to bring foes low and to empower their spawn.
The sibriex did not appear again until Mordenkainen’s Tome of Foes for 5e, which you have already read. I do not actually own that book myself, but it certainly sounds like they gave these unique and terrifying demons rather short shrift.
