Rift Attacks

The Rift Attacks or Rift War was a series of attacks targeted against the Empire that occured in the aftermath of the invasion and led to end of Standard Imperial Archetecture being used in city and Fort designs. Though the attacks are largely forgotten today, at the time they struck fear and panic across the Empire as portals began opeing above various locations across the world as if the sky itself was being torn open. From the portals came a swarm of destruction that in many cases obliderated whatever was below it. Despite the effort of the worlds best minds the cause and nature of the attacks were never determined and simply stopped happening after several weeks. 
The Arcane Alliance were noted to have believed the attacks to be magical in nature  though this fact is disputed today as it was at the time.