The Frog King is a secret, Easter egg boss in Loop Hero where there is no real benefit to gain from summoning and defeating it, unlike other bosses who drop Orbs of mortality and other useful resources. The wacky boss battle you engage in with this Easter egg boss once it has been summoned is basically a bit of fun that has been added by the developers. Our How to Summon the Frog King will show you how to make this boss spawn in Loop Hero.
In order to summon the Frog King boss you just need to cover the entire road of your loop with swamp tiles, this literally means every single tile must be covered. Once you have turned your map into a giant bog you simply have to wait for the Frog King to randomly appear at one of the tiles. All you then need to do is kill it, unfortunately because this Easter egg boss is so easy to summon the rewards that it drops when you defeat it are not worth the effort. The Frog King has 2 HP, but you will only be able to do 1 HP damage per hit and hitting this boss is not easy as its dodge is 99%.
Take a look at our How to Summon the Secret Frog King Boss
guide to see another way in which you can summon this Easter egg boss.