Due to the singular nature of Mortis Thule and the threat it represents to the Reach, and potentially to the Imperium at large, the Deathwatch has in place a standing order to be enacted whenever the space hulk is encountered.
Standing Order Epsilon Nine-Nine-Zero is to be carried out by any Deathwatch Kill-team that logs a new sighting of the space hulk, and there are very few circumstances that may override it.
In practice, only the direct order of a Watch Commander or an Inquisitor can countermand the instigation of Standing Order Epsilon Nine-Nine-Zero, and this is only known to have occurred in a handful of instances.
The standing order states that any Deathwatch force encountering an un-logged appearance of Mortis Thule must make a full augur sweep of its current form and configuration and, having done so, effect an immediate boarding action in order to map as much of its interior as possible.
This means the Kill-team must mount a hasty reconnaissance mission that can last anything from a solar hour to several solar days. The space hulk rarely remains in realspace for longer than this, though it did once hang in orbit above a now desolate world for three standard months.
The greatest danger is that the space hulk will return to the Warp with the boarding party still aboard, and thus every effort is made to glean as much advance warning of this as possible.
This means that the efforts of every astropath, Navigator, and Librarian nearby are turned towards sensing any alteration in the ebb and flow of the energies of the Immaterium that might indicate an imminent translation.
And so skilled are those psykers that serve the Deathwatch that it is comparatively rare for a Kill-team to be stranded on Mortis Thule when it plunges back into the Warp. Nonetheless, the Hall of Glory on Watch Fortress Erioch records the names and deeds of those that have paid the ultimate price for their valour in the service of Humanity.
- Deathwatch: Ark of Lost Souls (RPG), pg. 8