Vigil-Keeper Orestes Vahn
Orestes Vahn was the second of three apprentices to Vigil-Keeper Sevan, who held a fortified mortuary station on the Dominion's outer line for forty years. Sevan's rule was that the line is held until the dead beyond it are accounted for, and not before. When a Collective incursion took the station, Sevan ordered the apprentices to retreat with the burial rolls and continue the watch from their own ships. The order said the watch was not concluded.
The first apprentice broke it within a year. The third was killed in another action. Orestes is the only one still under the original instruction. The Dominion has tried three times to retire his commission. Three times he has produced the order and pointed out that no living officer has standing to countermand a dead one of higher rank from a previous chain. The argument is technically sound. The Dominion has stopped trying. The Unbroken Watch is the argument, still on station.
Source: codex:captain_orestes_vahn_lord