The secret organization battling the strange plague would be ancient, with long roots. Do the have political affiliations? Sponsors? Are they independents? Are they corrupt and bought, or independent agents?
I’m thinking the FBI with teleportation powers and some dark arts magics and cool scythes. Grim reaper secret police!
What’s their hierarchy and internal structure? Are there leaders, specialists, scholars, field agents, medics even? How large is the organization, where do they have offices? Do they make money somehow? Do they have dayjobs? Is “reaping” a freelancing gig?
Do they kill people? Probably. Do they have prisons for the undying? North Korea style concentration camps?