The Yuctan Incident in 666.M41 represented both the first known naval engagement between Imperial forces and the Necrons, as well as the first recorded instance of a Necron "harvest" of the entire population of a human planet. The Yuctan System was a sparsely inhabited Imperial star system close to the Eastern Fringe, and the site of the first naval encounter between the Necrons and the Imperial Navy. A minor Imperial fleet of 6 Escort ships and 1 Light Cruiser was destroyed, with the only survivor being a single Cobra-class Destroyer.
By the time a full Imperial fleet could be dispatched to the region to deal with what is now called the "Yuctan Incident," the Necrons had disappeared, and the system was found devoid of all human life. Although similar and unexplained events had taken place up to this time, the incident at Yuctan was the first fully documented account of a Necron "harvest" of a populated world, as they sought fresh subjects for their research into "apotheosis," the restoration of their consciousness to organic bodies.
For centuries the Explorators of the Imperium have known of the ancient civilisation of the Necrontyr -- a race that became extinct tens of millions of years ago. Little remains now of what must once have been a technically advanced Empire that spanned the galaxy. Any surface structures that may once have existed have long since disappeared from Necrontyr worlds. Their existence has been eroded by time. They and their works have crumbled to dust with the passage of eons. Like a thousand other dead civilisations and forgotten races the Necrontyr were of interest to few outside the dusty offices of the Explorator Archaeos. Then, suddenly and unexpectedly, a chance encounter brought the Necrontyr to the attention of the powerful and the great. Raiders marked with Necrontyr runes and indecipherable inscriptions of Necrontyr design were captured on a brief visual scan. It was a momentary visual record of the attack, yet it was a link in a chain of events that extended across the galaxy. It is a link to other raids, to other sites of destruction where raiders leave no trace and where records are wiped across an entire planet.
Until the latter 41st Millennium, almost all contact with the Necron's had been limited to battles fought on planets against small forces of Necron troops. From time to time reports were made of strange space craft encountered on the edge of newly discovered systems, which appeared derelict at first but when approached became active. More often than not all that remained of the Imperial craft which encountered these strange ships was the scattered wreckage of its remains, and the only clues to the nature of the enemy were garbled distress messages speaking of alien ships using weapons of quite terrifying destructiveness. Whenever such incidents were investigated nothing could be found of the perpetuators, and no useful evidence could be collected pointing to their nature or origin.
All this changed in the year 666.M40, when the first recorded incident of a Necron "harvest" took place in the Yuctan System close to the Eastern Fringe. Although sparsely inhabited, the system included an Imperial Naval depot. At the time of the attack the depot was being used by small Imperial squadron consisting of the Dauntless-class Light Cruiser Farsight and half a dozen escort craft. Squadron Farsight was undertaking a long range patrol, and was visiting Yuctan to refuel and re-arm, when it received distress messages from an Imperial colony on the outlying planet of Merida. The squadron moved to intercept and took part in the first recorded encounter between Imperial battlecraft and a Necron fleet. Only one of the Imperial craft survived to tell the tale (the Cobra-class Destroyer ON37452).
The Necrons suffered no losses. By the time a full Imperial fleet could be despatched to the Yuctan System the Necrons had disappeared. And of the human colonists in the system there was not a single trace. Since then there have been a further 27 recorded encounters between Imperial Navy ship and Necron raiding fleets. There are also anecdotal accounts of space battles between the Necrons and Eldar, Ork and even Chaos space fleets. All evidence points to the Necrons being old beyond the memory of anything living and their technology, although idiosyncratic, is superior to that of any other race, including the Eldar. In every encounter so far the Necrons have only been defeated by superior numbers of enemy ships, and wherever the numbers have been even the Necrons have prevailed. Fortunately for the Imperium all of the Necron fleets encountered so far have been small in size, and what should happen if the Necrons should ever attack in force remains to be seen.
Even more worryingly, the encounters that have taken place with the Necrons have occurred all over the galaxy, without any discernible pattern as to when or where the Necrons will strike next. What is more, the frequency of such encounters seems to be increasing, though how or why this should be happening is unknown. All that is known for sure is that the objective of these raids appears to be the seizure of Imperial citizens -- but for to what hideous use the captured men, women and children are put remains terrifyingly unknown. They simply disappear and are never seen again.