The Imperium Sanctus is the High Gothic name given in the Era Indomitus to the southern and western half of the Imperium of Man where the psychic beacon of the Astronomican is still visible to Navigators and psykers alike after the formation of the Great Rift.
The name is a hollow one, for even beyond the influence of the Great Rift, the dire edge of the times can be felt. Daemons and Traitor armadas have struck at Holy Terra itself during the Battle of Lion's Gate, and the expansive reach of the Warp Storms throughout the galaxy enable the forces of Chaos to strike without warning from nearly any quarter.
Warp travel and FTL communications are still disturbed within the Imperium Sanctus by the birth of the rift and the lingering effects of the Noctis Aeterna. But the Astronomican's continued transmission means that the worlds of the Imperium Sanctus are not as isolated from each other or as vulnerable to Chaos and xenos threats as those of the Dark Imperium.
The tithes continue to flow from most worlds, but although efforts have been redoubled to produce machines of war and to call forth regiments of troops, they are barely sufficient to the needs of the Imperium’s current state. Shrinking the Imperium’s size by half has not made it easier to supply, but instead has doubled the dangers to every world.
The two halves of Imperial space are now separated by the roiling Warp Storms that comprise the Cicatrix Maledictum, connected only by the passage through the rift known as the Nachmund Gauntlet. The gauntlet is book-ended by two worlds at either end, Sangua Terra on the side of the Imperium Sanctus and Vigilus on the Dark Imperium side.
Because of Vigilus' strategic position as one terminus of the only passage through the Great Rift, it became the source of conflict between Imperial, Chaos and xenos forces during the War of Beasts in the early years of the Era Indomitus.
However, the Imperium Sanctus certainly suffers from its fair share of conflicts and threats, as no place in the galaxy is now entirely free from strife as the power of Chaos waxes ever stronger.
The loss of resources from the worlds of the Dark Imperium have made every defeat suffered by the Imperium a greater one, but such is the strength of the Imperium Sanctus that the forces of Chaos are made to pay in equal measure.
- Imperium Nihilus: Vigilus Defiant (8th Edition), pp. 5, 11, "To Run the Gauntlet"
- Warhammer 40,000: Core Book (9th Edition), pg. 20
- Warhammer 40,000: Rulebook (8th Edition), pp. 50-53, 123, 134, 152, 164-166
- White Dwarf 35 (July 2019), "Indomitus Crusade Fleets," pp. 40-45
- Wrath & Glory: Core Rules (RPG) (2018 Edition), pg. 41