Sanctuary 101
Sanctuary 101 was a Sisters of Battle Fortress-Convent in the Vidar Secton on the Eastern Fringes, and the location of the first battle between the Imperium and the Necrons (Well the first to be known to the public at large). Unknown to the sisters their convent was actually built upon a Necron Tomb World, which at the time the Imperium didn't know was a thing (Minus the higher ups of the Imperium). The sisters were caught with their panties down when the Crons woke up in 897.M41. The Sisters battled the Necrons, but were inevitably defeated and slain to a (wo)man, and the battle came to be known as the "Massacre at Sanctuary 101". Though (SPOILER ALERT) if you count James Swallow's books as canon, there was a single survivor hiding in the desert (END SPOILERS). Video picts from the battle only showed the sisters being slain by shadowed figures. While those who reviewed the footage couldn't make out the attackers, the recordings, and the battle itself were the Imperium's first solid evidence of the Necrons' existence.
Sanctuary 101 was based off a battle report debuting the Necrons when they were first released. As the Necrons won they subsequently wrote that the sisters were wiped out The Necrons literally killed them all to the last model. This battle is the first instance of the Sisters of Battle being killed en masse, and arguably the start of the trend. However, unlike every other instance, it was not intended. The report took place back in the days of your dudes, when the outcome of battles could actually have a dynamic affect on some portions of the fluff. Having all the sisters wiped out was simply a consequence of losing to the original Necrons, who were homicidal killing machines, rather then the sick and cruel joke that it has now become.
The novel Hammer & Anvil, aka Round 2, did explain much about the battle including the fact the Sisters were never the Necrons' real target, it was really just two dynasties fighting each other, and the Sisters got caught in the cross fire. Then Sister Miriya blew up the nearby Necron Tomb Moon with a handful of Krak grenades in the trash chute. I wish I was kidding.
Another bit was that the High Lords, Alienhunters and AdMech had already known about the existence of Necrons for decades; it's just that every other encounter was able to be covered up due to a lack of solid evidence.