[@Arawak] There were three Supreme Hordelord invasions of the Empire before our time. The first was in the time of the Emperor, the second was in about 1000 IE, and the third can easily have been around 400 years ago. 

There were also a decent number of smaller Hordelord incursions. The Culling force couldn't stop them all afterall, especially in the more recent years as the Culling Force grew weaker.