The leprechaun king is a tougher version of a normal leprechaun, and appears wearing red instead of the normal leprechaun green.
If provoked, he will act as any other leprechaun.
Drop 20 red mushrooms to summon the leprechaun king. Whether or not the mushrooms are identified as safe to eat does not matter. Different interactions depend on how you've dealt with other encountered leprechauns. If you haven't ran into any leprechauns the king will say, I don't know you. Please be kind to my people.. There are no options available to the player at this point.
However, If you have been letting leprechauns live throughout the game, the king will give you the choice of new boots or improving a pair you own. If you decide on new ones, you will be able to choose any type the game offers. Be wary, as the boots will not be identified and you may end up with a very good pair of jumping- or levitation boots. Enchantments may or may not very, but are generally +3ish.