Magic: I Have a Demon Empire!

This world has magic, monsters, and danger, but also opportunities, mages, artifact smiths, potion makers, spellcasters, and all kinds of other professions. Yet somehow, a transmigrator ended up as the ruler of the food chain, an emperor-level monster who reigns over the waters. All he has is a not-very-useful System and a coffin. Most importantly, he's not the only transmigrator in this world?