Ordo Necros
The Ordos Necros is only five Inquisitors strong and shrouded in mystery. What's more it has an Ordos of unknown size specifically to watch over it. There are a number of theories as to the intent and area of study, here are just a few.
The most straight forward theory is the Ordos Necros studies death, simple as that.
The most fluffy theory is that the Ordos Necros are death cult assassins of so much skill and loyalty they were incited into the ranks of the Inquisition and have hence forth dedicated them selves to bringing the art of the sword to as many planets as possible and possibly designing and researching now methods of blade based combat. It also makes sense that other Inquisitors would want to keep an eye on them.
Based on the hazy link between 'Necros' and 'Necron' some believe the Ordos Necros study the Necrons, which would explain why they're so secret as so far no technology has ever been stolen from the Necrons.
- However, this connection can also be drawn from the Deathwatch (RPG), where there exists a faction within the Ordos Xenos called the Dead (ie: Necro) Cabal which consists of only Five members. Their purpose was not explicitly to deal with Necrons, but a mysterious Dark Pattern of events which are prophesied to culminate in the "End... (ie: Necro) ...of all things". Though they were passed off as potentially dangerous eccentrics, which makes sense that other Inquisitors would be monitoring them carefully when things start going as predicted.
- The awakening of the Necrons was just one event that they predicted in advance and they may in fact have some connection to the missing Ordo Chronos, as two of the members have no history at all, one of them identified by a crystal hourglass and both just seem to have "appeared" when necessary knowing exactly what/when/where is about to happen.
Finally the least likely but perhaps most interesting theory is that the Ordos Necros studies the Legion of the Damned, each as secretive as the other, it would make sense for the Inquisitors to want to keep track of the comings and goings of these mysterious zombie/ ghost/ space marines.
