Sangua Terra was an Imperial War World in the Segmentum Obscurus that represented the Imperium Sanctus end of the Nachmund Gauntlet, the only known open passage through the Great Rift to the Imperium Nihilus.
It contained large deposits of blackstone ore within its planetary crust that had been altered by unknown xenos technology to interface with a similar blackstone concentration on the neighbouring world of Vigilus.
Because of this location, Sangua Terra became one of the most strategically valuable locations in the galaxy.
Early in the Era Indomitus, the world was a focal point of conflict between the forces of the Imperium and the servants of Chaos commanded by Abaddon the Despoiler during the campaign known as the War of Beasts.
Before the opening of the Great Rift, Sangua Terra's star system had represented a major way point along one of the Segmentum Obscurus' stable Warp routes. However, when the Great Rift opened, Sangua Terra was enveloped by its edges and was assaulted by waves of daemons.
Lord Commander of the Imperium Roboute Guilliman despatched the Castellans of the Rift, a Space Marine Chapter raised during the Ultima Founding, to drive back the hordes of daemonic war machines and Renegade Knights blighting the Nachmund Gauntlet. As part of that force, 2 companies of the Castellans of the Rift made planetfall on Sangua Terra to drive back the servants of Chaos.
In 025.M42, Abaddon the Despoiler and his Black Legion were driven from the surface of Vigilus, preventing its destruction. Yet word soon arrived of an attack on Sangua Terra.
The report mentioned a giant voidship approaching in low orbit, a craft that fit the description of a dire threat from ages past -- the Planet Killer.
Creation of the Nachmund Gauntlet
Noctilith stone, also called blackstone, had a peculiar property that, to those who understood the nature of the cosmos, made it more valuable than any other resource in the galaxy.
Blackstone was Warp-resonant, and could be charged either to attract or repel empyric power.
The spear-like deposits in Vigilus' crust had been polarised to repel Warp energy by some ancient xenos technology. It was these that were holding back the Great Rift around Vigilus -- and indeed, by creating a channel of anti-Chaos force between the Imperium Nihilus and the Imperium Sanctus, forming the Nachmund Gauntlet itself.
The planet of Sangua Terra had the exact same spears of blackstone in its crust, held in a strange black suspension that meant they always pointed towards Vigilus.
The anti-empyric field that thrummed between these spears kept the Nachmund Gauntlet open.
If Abaddon the Despoiler succeeded in destroying that esoteric resource, the Warp Storms around the Nachmund Gauntlet would close in, and the corridor of safe passage would be subsumed completely, leaving the Imperium Nihilus to the mercies of the forces of Chaos.
It was for this reason that the Despoiler and his Black Legion were so determined to seize or destroy Vigilus and its counterpart on the other side of the gauntlet during the early years of the Era Indomitus.
- Warhammer 40,000: Rulebook (8th Edition), pg. 53
- White Dwarf (October 2017), "Eavy Metal - Create your own Chapter," pg. 118
- Imperium Nihilus: Vigilus Ablaze (8th Edition), pp. 26, 44, 117
- Vhane Glorious No. 007