The Storm King is an over-sized Storm Beast with a large multi-spear attack which homes in on the player's location.
Post purification, heroes are offered to the Storm King. The Storm King is worshiped by Shadowmen. They believed the beast brought forth the rain and was ruled by the God Storm King.
One strategy is to attack the Storm Beasts and then focus on the Storm King. There is a sword that is crucial for melee fighters, called the Stormruler. It is found at the end of the monolith forest. Other than that, spells and arrows work well on this boss.
Another way to defeat this boss without much hassle is to grab the sword and then run into the house which is located a little deeper into the area. While in the house, it is possible to safely attack the Storm King and the Storm Beasts without fear of being hit.
If the house is too vulnerable a spot to defend in, the largest, tallest pillar in the middle of the island will suffice as it can block the Storm King's projectiles and it is also in the flight path of the boss itself, allowing two or three opportunities to attack.
Soul Arrows are very effective.