A World That I Rule

The protagonist is sucked in to a weird whirlpool in a river and somehow ends up in a island in another world with native elves that wear leaf clothes, have the knowledge of an new born, and have sex like rabbits. That also have a body as weak as butter.
He decides to become the king of the place and rule with an iron fist and teach the naive elves cruelty, evil and watch as they kill each other.