The order was established centuries ago, probably during the same time as in the real world (18th century) but with the help of otherworldly Alexander of Brennenburg. It's apparent mission was to serve the interests of Prussian nobility and the Prussian Kingdom itself.
Membership in the order was apparently hereditary, with Alexander noting in one of his Memory Capsules how the other members of the order pretended that he was a descendant of an "original" Alexander who founded the order a century prior. On the other hand, worthy individuals could have been accepted into the order and promoted within via recommendation from the more senior members, such as was the case with Wilhelm von Gerich.
The Order was intimately tied to the daily politics of the Prussian Kingdom, with Alexander noting how they 'spewed their politics' at each other.
Interestingly, the Order seems to have had some dealings regarding Napoleon Bonaparte, as Alexander notes that: The Black Eagle fears me and after Napoleon's defeat it is only a matter of time before they will demand my head on a platter. This could either mean that either the Order wanted Napoleon on power and his defeat was somehow pinned on Alexander , or that he represented a grave danger for Prussia and Alexander was somehow hoped to protect the kingdom, with the defeat of Napoleon making Alexander no longer useful to the Order.
It is unknown what became of the order after the events of Amnesia: The Dark Descent, though it can be assumed that it continued to exist for some time. In the real world, the Order of the Black Eagle continued to operate into the 21st century, with the German Military police (Feldjaeger) being the latest bearers of the emblem.
The Order of the Black Eagle is mostly relegated to the background, being only mentioned in various texts and other, more esoteric information mediums.
The only visible symbols of the order are shields and statues of eagles that can be seen throughout Brennenburg castle.
As with the previous instalment, the information about the order is exceedingly scant and merely hinted at. Oswald's Mansion features shields bearing the insignia of the Black Eagle, and the cemetery at St.Dunstan's Church features a few graves which also bear the insignia.
It is still unknown whether the Order will make some kind of reappearance in the third game.
- While in real life, the order was more of a commendation of honor in the Prussian Kingdom, in the Amnesia Universe it appears to have been a full-fledged political organisation which dabbled in European politics.
- The order seemed to be aware that Alexander wasn't a human, with them greatly fearing him but also tolerating him as well. It is unknown weather the Order was aware of the Orbs and/or other aspects of Alexander's more supernatural dealings.