Heretic

Three brothers, known as the Serpent Riders, have used their powerful magic to possess seven kings of Parthoris into mindless puppets and corrupt their armies. The Sidhe elves resist the Serpent Riders' magic. The Serpent Riders thus declared the Sidhe as heretics and waged war against them.

The Sidhe are forced to take a drastic measure to sever the natural power of the kings destroying them and their armies, but at the cost of weakening the elves' power, giving the Serpent riders an advantage to slay the elders.