A World That I Rule

The protagonist is sucked into a weird whirlpool in a river and somehow ends up on an island in another world with native elves that wear leaf clothes, have the knowledge of a newborn, and have sex like rabbits. That also has 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, and evil, and watch as they kill each other.