There have only been two instances in history when monsters banded together under one banner. This is not like a monster wave that is because of a dungeon exploding in excess energy or when powerful monsters suddenly appear in a territory. This was a full-blown army made of monsters of different races and even personalities.
The two instances that Adrian is talking about is when the Titans and the Gods were at war which was called the Titanomachy. The second instance is called the Gigantomachy and it is when the Gigantes rose up and challenged the Gods of Olympus for their seat at their divine thrones. These two instances were not recorded in history as it was a very brutal time in mortal life that the gods of Olympus even casted a wide memory erasing spell.
