Pruning Event

A pruning event is a disastrous event that can occur within the House. Although predominantly affecting sublevels, it can occur anywhere within the House.
The House permits the creation of endless levels, but this unlimited propagation will quickly result in the levels overlapping and destroying the House. Pruning events allow for the removal of "diseased" areas or areas that have become "overgrown", enabling the rest of the House to prosper. These areas are recognized as becoming dead ends, and fresh Ether from the Abyss ceases to be supplied.
A pruning event tends to occur once every 250 years, though this can change to only a few years, something that has only happened once since the end of the Age of Ancients. Almost all recorded pruning events are small, affecting less than 250 square miles, but the largest confirmed pruning event annihilated nearly 300,000 square miles in 2654 BV.
Some Imperial Scholars believe that Ragnarök, occurring at the end of the Age of Ancients, was a sort of pruning event that affected the entire House. The sheer scale of Ragnarök has never been seen before or since, leading to some doubts about this hypothesis.

Manifestation

Pruning events are foreshadowed by the stagnation and corruption of the affected area's Ether. Plants and animals start suffering from the effects of the stagnating Ether and either die out or attempt to flee the area. Weather patterns change sharply, resulting in increasingly dangerous weather systems. Any humans in the area will suffer the same effects as any other animal.
As the stagnation becomes more severe, the land itself begins to fracture. Almost if not all avenues of escape become cut off, and the area becomes isolated from the rest of the House. Any affected area on the main level of the House sink to sublevels and from there, fall into the Abyss. All of the surviving lifeforms in the area are annihilated.