Books of keeping

According to the monster manual:

"The first yugoloths were created by a sisterhood of night hags on Gehenna. It is widely believed that Asmodeus, Lord of the Nine Hells, commissioned the work, in the hope of creating an army of fiends that were not bound to the Nine Hells. In the course of making this new army, the hags crafted four magic tomes and recorded the true names of every yugoloth they created, save one, the General of Gehenna. These tomes were called the Books of Keeping. Since knowing a fiend's true name grants power over it, the hags used the books to ensure the yugoloths' loyalty" (pg. 311).

It goes on to say "Petty jealousy and endless bickering caused the sisterhood to dissolve, and in the ensuing power grab, the Books of Keeping were lost or stolen. No longer indentured to anyone, the yugoloths gained independence, and they now offer their services to the highest bidder" (pg. 311).

The world of Brelyria expands on this lore somewhat. Firstly there was only one original tome instead of four, but this tome only contains the names of yugoloths alone. Secondly, while it was rumored that Asmodeus had the work commissioned, the truth is that the sisters enticed both Asmodeus and one of the more powerful demon lords to with their creations, offering both Devils of the Nine Hells and Demons of the Abyss opportunity to use the mercenary race.

Their true goal, however, was to use the yugoloths to discover the true names of the most power Archdevils and Demon lords in the multiverse, infiltrating both armies with their mercenaries. Over eons these yugoloths created two new books of keeping--one for the Nine Hells, and one for the Abyss. How the yugoloths accomplished this is not completely known, although many have tried to discover their secret.

Yugoloths are still very protective of these books as they represent both their greatest strength and weakness.