I host a group of players that participate in a Dark Heresy/Deathwatch crossover game. Very early during the original DH campaign (late in 2008), the PCs found themselves far out on the Eastern Galactic Fringe, at the furthest reach of the Emperor's Light, south-east beyond the realm of Ultramar. There floated a Dead World, its name long forgotten. Upon this Dead World was a vast alien necropolis that resembled intricate electronic circuitry when viewed from orbit. The necropolis was, in fact, a massive machine that, when activated, creates a galactic psy-barrier. Ancient and knowledgeable Explorators had no clue to its ultimate purpose, but knew instinctively it could bode nothing but doom, and dubbed this alien machine The Nemesis Engine.

The Nemesis Engine (an intricate multi-node device constructed by the ancient Necrontyr) is actually a series of planets that trail along the corner of the Eastern Galactic Fringe. As each node of The Nemesis Engine is ignited it creates a linear barrier, bifurcating the galactic corner from the Emperor's Light. The reach of the Nemesis Engine's power is unknown, but the PCs surmised it could be used to effectively cut the Tyranid Hive Fleets' communion with the power of the Hive Mind, much like kicking an ant hill and killing the queen all in one go. Meddling in ignorance, the PCs ignited many nodes of The Nemesis Engine, abandoning the galactic south-east to its doom in order to protect the greater Imperium.

This was long, long ago, before the discovery of the Calyx Expanse and the subsequent Angevin Crusade. Unknowingly, the PCs were responsible for the Jericho Sector's long Age of Shadows. At least that's how I've chosen the timeline to flow based on the original campaign.

The Shadow in the Warp, created by vast Tyranid fleets, agitates the Necrontyr as they slumber deep within their Tomb Worlds, causing them to rise in response to this strange psilence. The Nemesis Engine nullifies the power of the Hive Mind, but it's harmonics resonate throughout the galactic region as an alarm, a wake-up call for nearby Necrontyr to rise from their tombs and reconquer the galaxy.

Present day...
The Achilus Crusade endeavors to bring the Jericho Reach once again under Imperial rule. In my campaign, the Orpheus Salient is beset by the leading tendrils of the Hive Fleets once bound for Ultramar but sent into disarray by the original ignitions of The Nemesis Engine. DH and DW PCs find themselves with some terrible choices to make; continue in the footsteps of long forgotten Acolytes from a bygone era, ignite more nodes of the Nemesis Engine to save the Orpheus Salient from the Tyranid threat, a threat within reach of the Well of Night, the Jericho-Maw Gate and the Segmentum Obscurus beyond (Terra's veritable back stoop) and buying the Imperium another half dozen or more millennia of protection), while at the same time waking the slumbering Necrontyr centuries sooner than canon tells us, possibly turning defeat of the Salient from one enemy over to another, again within easy reach of the Gate.

There is no clear solution. Damned if they do, damned if they don't. Two unknowable foes. Political mire as various Imperial institutions and personae vie for the supremacy of their own agendas. Near-limitless possibilities for my campaign.

Very nice. Also leads to the possibility of delving deep into one of the Nemesis Planetary Nodes to set off a detonation that will trigger the others to detonate in a massive chain reaction that'll slow the rise of the Necrontyr.

OR

Stopping a Daemon Prince of the Iron Warriors, once one of the Chief Librarians of the fallen legion, who has discovered a way that the Necrontyr machine could be sabotaged, reversing the Psi-barrier and opening huge tears in the barrier between real space and the Sea of Souls. The Hadex Anomaly is just a test run using one of these Nemesis Engines, but he's almost cracked the art of triggering a chain corruption that will flip every engine into warp rift mode, dooming the Galaxy.

OR

The Tau have discovered one of these worlds. Through careful technological investigation, they've dertermined that it is possible to vary the level of Psilence the Nemesis Engine can produce, allowng them to selectively block brighter siglas in the warp. They intend to use the knowledge of blocking the Astronomican to blackmail the Imperial forces in the reach into a ceasefire and peaceful subjugation.

OR SO MANY MORE POSSIBILITIES!

Okay, I might steal this.

These are awesome.

My Kudo's for thinking of this. Ill definatly use them, albeith in smaller versions, on my plannet based story.

It might also be funny to see the Hive having adepted to these barriers over the centuries. as the tyranids constantly evolve to keep on going. One means of retaining control is through sub light signals of the infrared spectrum. given that the hive fleet tendril is based on billions of ships, if they direct their ( flashlight ) onto a region all at the same time they could send some sort of evolved morse code in the light. wich will result in the tyranids viewing it to know what must be done. although its not as omni present as the hive minds spycic profile would heve been, it might rally the troops and become a serous treath again. the first signs of this transformation could be silvery rimmed armor plates, where the silver is a tissue edge that recieves and emits the infrared flares.

giving your players another thing to go after is the research into this wich could then lead to a means of using it against the chaos, or Tau medling, and perhaps against the hive itself. Another Jet DOOMING choice to make for it is herecy to use and co-excist with these tyranids to preserve the galactic core… or now is it?