The Drowned God was Greenseers and the Children of the Forest created the Deep Ones:
https://thesuddenstorm.wordpress.com/2018/03/12/the-truth-behind-the-drowned-god/
A theory of mine I wrote about a year ago. If you like the ancient mysteries of the World of Ice and Fire book, this is for you. It covers the lost civilizations and their structures, such as the fused black stones and oily black stones.