The Nasgareth may look like elves, but instead were created by an entirely different divine being, which admired the elvish looks and traits, but wanted to create his own army instead. One without Love.
Thunder and lightning is the sign of their shamans hunting and killing demons. Children's souls must be protected for their souls are not firmly in place yet and are easy for demons to take hold of. The shamans must hunt and destroy them and each break of thunder is another death of a demon hunting for souls.