The Ordo Sepulturum are surprisingly not a black metal band, but a minor Order of the Inquisition, dedicated to fighting plagues such as the Obliterator virus and the Zombie Plague. However, they were formed primarily due to the increasing presence of Plague Zombies (created thanks to Typhus and his Plague Fleet) around the Eye of Terror, and as such, they have obtained the moniker of the Zombiehunters, mirroring the Daemonhunters, Witch Hunters, and the Alienhunters.
They've also had next to no material published about them, having only been mentioned three times. First in an Inquisitor sourcebook that's now out of print, then again in The Death of Antagonis, and finally in Cadian Blood. A pity, for a group with such a cool concept behind them.
Naturally, as with all things that happens in /tg/, some anons have remarked that the Ordo Sepulturum's chamber militant should be a small elite division of Tempestus Scions analogous to the Blackwatch. Admittedly, the concept of having Imperial Guard Stormtroopers be the Chamber Militant of something is awesome.