In the South West region of the Chalnath Expanse in the Ultima Segmentum, by the K’ail Plant system, the Chyron system has become the next target of one of the many tendrils of Tyranid Hive Fleet Leviathan.
The Chyron system represents one of the gateway systems to the larger Chalnath Expanse where both Tau, Chaos, and Imperium have beach heads and strategically important systems. The Chyron system is important due to the biomass reserves being enough to fuel any tendril to many of the other key systems.
The appearance of this tendril has forced a truth with the imperial forces on the planet, and the Tau science exploration fleet that happened to be in the area when the first Tyranid scout vessels entered the system.
Unbeknownst to these defending forces, the Necron have located a relic of great importance on Chyron III, the same planet where the defenders will be making their stand against the Tyranid tendril.
The Necron see the Tyranid as the perfect distraction for them to slip in and grab their relic, without committing a full siege force to pry it from the Imperial forces that have been defending the planet.
With space forces from the defenders keeping the Tyranid at bay, the ground forces must hold their ground in Hive City Terminus Prime on Chyron III to prevent the tendril from establishing resupply pillars. If they don’t, there will be no stopping the tendril from taking the rest of the system.
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https://wahapedia.ru/wh40k10ed/the-rules/tyrannic-war/#Tyrannic-War-Campaign