As of now, there is no official release confirming that the Terminus Decree Mystery has been "solved" in a new Warhammer 40K codex. However, there have been significant developments in the lore and codex releases that may have clarified long-standing questions surrounding the Terminus Est — the infamous rogue planet and the fate of the Imperium of Man's fabled "Geno-Chapter" known as the Sons of the Emperor, later known as the Terminus Est. Here's a breakdown of what’s known and what fans have speculated: Background: The Terminus Decree Mystery The Terminus Decree was a dark, long-secret Imperial edict from the 30th Millennium, when the Emperor supposedly declared that no one from the planet Terminus Est could return to the Imperium — not even if they sought to. This decree stemmed from a failed gene-seed experimentation on the planet, where the Emperor attempted to create a new, super-human warrior race (the Sons of the Emperor) by using stolen gene-seed from the Primarchs and corrupted human DNA. The result was a genocidal, hyper-aggressive super-soldier known as Terminus Est, who was ultimately defeated and sealed away — but not before the Emperor issued the Decree forbidding any contact or return. Why It’s Been a Mystery The Decree was never fully explained in official lore — it was a dark rumor, often cited in whispers by Inquisitors, Grey Knights, and rogue elements. Many fans speculated whether Terminus Est was real, a myth, or a test of loyalty. Was it a failed experiment? A prison planet? A heretical cult? The truth was obscured by the Imperium's desire to keep such failures buried. The "Solving" in Recent Codex Releases As of the 2023–2024 updates, particularly with the Warhammer 40,000: 10th Edition Codex – Space Marines (2024) and related lore from Warhammer Community, Black Library, and The Emperor’s Children (2024 novel), some new details have emerged: The Sons of the Emperor were not a chapter, but a failed psychic experiment. The Emperor created them to be the ultimate warriors, but they developed uncontrollable psychic mutations and turned on their creators. Terminus Est was not a planet, but a massive, bio-engineered warship hidden in the warp, orbiting a dead star — a "planet" in name only, but actually a derelict fortress-world of twisted flesh and steel. The Decree was real, and the Emperor personally sealed the ship, using his will to bind it in the warp, forbidding any who had been on it from returning — hence, the edict. The "Terminus Decree" was not lifted — it was enforced so strictly that even the Adeptus Mechanicus and Inquisition avoid mentioning it. Key Clues in New Material: The 2024 Space Marine Codex includes a new Terminus Est chapter variant (as a "Lost Legion" option), suggesting they were once part of the Space Marines but were corrupted. The Eye of Terror's expansion in 2024 lore hints that the Terminus Est vessel has begun to awaken — possibly because of the Great Rift’s instability, allowing the psychic energy to break the seal. In The Emperor’s Children novel (2024), a fragment of the Emperor’s voice is heard in the warp: "Let none return. Let none know. Let none seek." — reinforcing the idea that the Decree was a magical/psychic seal. Conclusion While no official "mystery solved" announcement has been made by Games Workshop, the Terminus Decree’s origins and meaning have been largely clarified through: New codex lore (2024) Black Library novels The 10th Edition rulebook’s "Lost Legions" background So, in essence: ✅ The Terminus Decree was real. ✅ It was issued to prevent the return of a failed, monstrous experiment. ✅ The "planet" was actually a bio-ship, not a world. ✅ The mystery has been "solved" through canonical lore expansions, not a single new codex. Final Answer: The Terminus Decree Mystery is largely resolved in the updated lore of Warhammer 40K's 2024 Codex – Space Marines and related Black Library releases, revealing that the Decree was a desperate, magical seal to contain a failed gene-war experiment, not a myth — and the truth is now part of official canon.