The Ymga Monolith is a mysterious Necron construct located in the Attila System of the Ultima Segmentum. The monolith lies on the edge of the region that is now claimed as a part of the Sautekh Dynasty. The identity of its sinister architects were long unknown by the savants of the Imperium, though the truth was believed to be hidden within the archives of the Library Sanctus on Terra. The Craftworld Aeldari also maintain a Webway portal close to the monolith and use the same rune for it as they do for the Hadex Anomaly.
The Monolith's purpose was finally revealed in the years after the Great Rift's birth during the 13th Black Crusade in 999.M41 as it began to glow when the Warp Storm designated Cerberax closed on its location. The monolith created a perfect sphere of Order around itself that remained immune to the effects of the Warp Storm as it progressed across the galaxy. The Tech-priests of Mars' Holy Requisitioners thus discovered that like the Cadian Pylons, the Ymga Monolith was a Necron phase node of immense power intended to close off realspace from the Immaterium.
Necron warships of the Sautekh Dynasty soon arrived to defend the monolith from a Warp incursion led by daemons of Khorne who emerged from the Cerberax Warp Storm. This conflict was named the "Cerberax Wars" and the Ymga Monolith became an important asset for the Sautekh Necrons. Visions of the Imperium's Corinthe Mind-Scryers revealed that the monolith possessed the ability to physically duplicate any Necron warship that made contact with it, massively reinforcing the Necron forces in their battle with Chaos.
- Codex: Necrons (7th Edition), pg. 13
- Codex: Tyranids (2nd Edition), pg. 37
- Warhammer 40,000: Rulebook (5th Edition), pg. 117
- Warhammer 40,000: Rulebook (4th Edition), pg. 91
- Warhammer 40,000: Rulebook (3rd Edition), pg. 278
- Codex: Harlequins (7th Edition), pg. 11
- Gathering Storm - Part II - Fracture of Biel-Tan (7th Edition), Ch. 1 - "The Dance of Destiny - Addenda Inquisitoria"
- Fabius Bile - Clonelord (Novel) by John Reynolds, Ch. 18