The Lost War occurred 8547 years into Riariti's past (as of the start of the comic), marking the loss of much of the worlds knowledge. It was also the cause of the extinction of several races, including the Ancients.
No one quite remembers what caused the war since everyone seems to have a different story. What is known is that it somehow played a part in a fight between the goddess Fate and another god whose name has been stricken from history. The war involved every race and every nation, becoming so horrible and causing so much widespread destruction that even the Titan Dragons were forced to move, the only recorded instance of them doing so.