The Varangian Guard are the bodyguards of the Byzantine Emperor, existing in all of the game's timeline except in 867 AD, where they need to be created. The Varangians are of Norse origin with many of them coming from Scandinavia, those who have nothing to inherit in their homes usually found employment with the Guard and were able to find glory and pay by serving with the Emperor.
In Game Edit
The Varangian Guard is not a holy order, thus do not require piety to maintain them but gold. However, as they are vassals of the Emperor, they can only be called up by the emperor, and can be integrated to the personal levy of the Emperor without any problem whatsoever. Unlike normal vassals, they do not have an opinion penalty when kept up after a certain time and unlike holy orders, the Varangian Guard can engage rebels of the same religion and would usually defeat the rebels.
However there are slight caveats to the Guard. Firstly, they require gold to maintain not piety but the Empire can take the economic hit. Secondly, unlike holy orders who numbers more than 7000, the Guards are only 3500 in numbers, which is fine in early game, but in later game they would need to be integrated to the personal levy to be more effective in combat as many armies would outnumber them.