At an unknown date in the far past: The First City falls. Its name and location is unknown, aside from it having been a mighty city somewhere in Ancient Mesopotamia. It is speculated that it might have been the Sumerian city of Uruk or the Akkadian/Hurrian city of Nagar.
Ca. 1335 BC: The Second City, Amarna, the capital of Ancient Egypt during the reign of pharaoh Akhenaten, falls.
Ca. 300-400: The Third City falls. Its exact name and location is unknown, but it is known that it was Mayan. Possible candidates includes Chichen Itza, Tikal, or Calakmul.
1254: The Fourth City, Karakorum, the capital of the Mongol Empire falls. The events of The Silver Tree takes place here.
1862: The Fifth City, London, the capital of the British Empire falls.
Mask of the Rose takes place shortly after the fall of London, starting about six months after, and ending a full year after.
1888: Sunless Sea begins.
Ca. 1890: Fallen London begins.
One possible future
At some point after 1888:
Sunless Sea ends with the Captain and the Merchant Venturer journeying to Avid Horizon and successfully opening the Cold Gate, thereby discovering the High Wilderness.
Following the discovery of the High Wilderness, the New British Empire, under the leadership of Queen Victoria, starts a large scale colonization effort and spreads itself to the stars.
1906: Sunless Skies begins.
Ca. 2200: The god of the afterlife, the Sapphir'd King, is finally murdered after decades of being eaten alive by the Spider Senate. His usurper then chooses one of three destinies:
Become the ruler of an undead empire.
Take the Sapphir'd King's title as god of the afterlife
Permanently ensure anarchy in the afterlife and allow the dead to come back to life whenever they want
Oh, and France manages to establish colonies in the High Wilderness.
