Imperial Investigation Bureau

The Imperial Investigation Bureau is based on the Censorate/御史台, a high-level supervisory agency in Ancient China that was hated and feared by civil and military officials alike, and it was especially powerful and effective during the Mongol-ruled Yuan Dynasty, as well as in the following Han-ruled Ming Dynasty. I've taken (a lot of) artistic liberties and combined them with the 锦衣卫/Embroidered Uniform Guards, who directly served the Emperor and eventually became an imperial military body that was almost like a private secret police service. They were given autonomy in arresting, interrogating, and punishing anyone, and also tasked with collecting military intelligence in wars. They wore very distinctive embroidered uniforms and were also widely feared and hated. Neither of the agencies used masks, I took that idea from 羽林卫/Feather Forest Guards, a branch of the Imperial Guards in the Tang Dynasty that wore steel masks to scare the enemy and also to hide their identities. Wearing metal masks for the effect of intimidation has a historical tradition stemming from the pre-Qin era, but the Imperial Investigation Bureau doesn't actually wear metal masks, mostly because I can't justify how they would avoid stinking of sweat while on incognito mode if they're wearing metal masks on their faces while running around.

As for moving around in trees, this has no historical basis. I'm drawing from the wuxia trope of jumping around in trees or bamboo groves. But also, Luoyang is a city with a long history, and many of its trees would have been there for at least decades. As a result, the Imperial Investigators would have an easy way of incognito travel via the large trees that are already in the city, most of which are large enough to hold up to three grown men of average height and weight. However, since the Investigators can be a bit petty and territorial, they rarely choose to hide in the same tree as their colleagues. Traveling through trees may seem counter-intuitive for the purpose of subtlety, but actually serves their purpose much better compared to running across tile rooftops, which would inevitably make a lot more noise and be more visible. 

The reason why Imperial Investigators all choose to do parkour instead of taking a carriage or something is partly out of necessity for surveillance and tracking needs, and partly because the Bureau's founder was highly skilled in parkour and martial arts, but after he was promoted to the head of a new institute, these skills were no longer necessary. As he felt that it was a waste of his training, and also because he was a drama queen, he started the tradition of jumping around in trees. Most civilians and lower-ranking officials don't know about the tradition, and assume that the constant rustling in tree branches is due to winds and also the presence of large amounts of magic (and also the Emperor) in the capital city.