Medimortimers.
by @Lapin

Medimortimers : A combination of "medicus" + "mortis" (using "m" instead of "s") + "er" suffix, meaning death doctor. Pronounced the English way: mɛdəmɔ:rtɪmɚ or mɛdəmɔ:rtəmɚ 

Medimortimers are an organization of underground scientists and doctors who use science or magic or a mix of the two to reverse death. They also have other goals such as to advance medicine, prolong human lifespans, heal injuries deemed too severe to repair, and strengthen cells in order for humans to heal and endure pain and damage better. They are extremely secretive and don't allow their members to leave. 

Their laboratories are located in both the wight and human world(s) alike, found in remote areas, well-hidden or disguised as abandoned buildings with the true part located underground. They primarily use underground tunnels to travel (to and fro these places), but they also use magic portals.

Medimortimers are completely unknown to human governments and have no cultural or media presence; and for the most part, their activities are not known to wights either. However, they are under the radar of grim reapers and risk having their work destroyed and their lives ended by them. 

Due to being targeted by reaper, there is a side goal of Medimortimers--that is, developing technology that disrupts the reapers' ability to detect other beings and to "halt" them through doing so. Technically, they already have...

v

Better/more info concerning the death experiments found here.