The Jurchens aka Jin were a Tungusic people who inhabited the region of Manchuria until the 17th century, when they adopted the name "Manchu". They established the Jin Dynasty (1115–1234) between 1115 and 1122, which lasted until 1234 with the arrival of the Mongol Empire.
Jurchens in Agent of Byzantium
The Jurchens were a nomadic people, originally from the far east, who in the 13th and 14th centuries migrated westward all the way to Europe, appearing in force on the steppes north of the Danube where they displaced earlier tribes and became the latest threat to the Roman Empire on the river's south bank.
The Byzantines did not know (and did not greatly care) what had driven the Jurchens from their original home and made them migrate so far west. What mattered was that they were a dynamic and capable people who presented an increasing threat to the Empire.
The Byzantines remembered that there had been many earlier such nomadic invaders – Huns and Avars, Bulgars and Magyars, Pechenegs and Cumans - and that some of them in their time did manage to breach the imperial borders and on occasion even threaten the capital Constantinople itself.
In the case of the Jurchens, moreover, not only were they courageous and skilled warriors, but their Shamans had some kind of magic enabling them to see over a distance, which gave a significant advantage in fighting the Imperial forces.
It was fortunate for the Empire that in the year Etos Kosmou 6814 (AD 1305) its warrior Basil Argyros succeeded in discovering that this Jurchen secret was no magic, but a clever use of lenses which the Shamans put in front of each other inside a tube, enabling the holder to see distant things clearly. Argyros also managed to steal one such tube and bring it back to the Empire, to be duplicated.
This coup earned Argyros promotion to the elite rank of magistrianos, which began his long and illustrious career as a spy and secret agent. Depriving the Jurchens of their monopoly of this remarkable invention made life easier for the Roman forces on the Danube Frontier, though it by no means put an end to the Jurchen threat.
The Jurchens' geographical expansion in this series mirrors that of the OTL Mongols. In OTL, the Jurchens themselves never showed any inclination to migrate westwards, their energy being directed southwards into China. Turtledove does not specify the apparent butterfly effect, whereby the POD of Muhammad turning Christian and Islam never appearing eventually led to the Jurchens migrating all the way to Europe. In OTL, though the expanding Muslim Empire never seriously threatened China, the Chinese did need to pay attention to this major phenomenon to their west. With no Muslim Empire arising, China might have been able to direct more resources to its northern border - which would have directly affected its Jurchen neighbors.
- See, e.g. Agent of Byzantium, loc. 34, ebook.