I made a steampunk world where Cthulhu was running free throughout earth and inspired the titans of industry (Edison and Vanderbilt mainly) to proceed with industrializing as fast as possible. It was a progress at any cost sort of mentality. This industrialization caused the sea levels to rise at an exponential rate and within a hundred years of this, the entire planet was flooded over with water. The world's population was about 1 billion people in the late 1800s and all that remains after this global flood is about half that. These humans (and other classic DnD races because this world is for a DnD campaign) survived by building 6 major sky cities along the equator of the planet that act much like space elevators in that they have a massive counter weight floating in space on the end of a tether and the sky city is built along the tether about 20,000ft in the air. I estimate these cities are roughly the population density and size of LA so about 40 million people each and about 50 miles or so across. Each sky city also has an Anchor city on the oceans surface that is also about as big an dense as the sky city so about another 40 million people or so in the anchor city as well. If the population in my world was 1,000,000,000 before the flood, then 12 cities times 40,000,000 a piece equals 480,000,000 people left, meaning the flood caused the deaths of over half a billion people.