Name: Great Flood / Endless Ocean
Circles: Water / Summoning
Description: Each turn existing ocean tiles expand by one pushing shoreline further inland. Any tile adjacent to mountains or cities is converted to shore as the ocean approaches them but all other tiles are eventually consumed by the expanding ocean. Once the enchantment ends for any reason the process is reversed with ocean tiles slowly turning back to their original state. While the spell is in effect each round there's a chance for neutral water unit(s) to be summoned from the plane of water to one of the ocean tiles.

Note: This is basically a turtle player spell. It blocks building of new cities and punishes those with a lot of troops without removing existing cities or units. Since this affects also your cities it's more of an endgame spell where you can go around razing/conquering enemy cities while making sure new ones are not built.

I'll say this is an end-game spell. For me this spell would have to have a sizable upkeep.