The ring of lightning is a powerful spell that kills all enemies on screen that can be killed with the magic level the user currently has. Whenever the user gains the next level of magic, the ring will upgrade to that level too. It requires full Magic Points to use and consumes all of them upon activation.
Twinsen gets the ring of lightning after he completes the recipe of Bersimon in the cauldron in his tent, with the missing ingridient: The Pearl of Incandescence. Twinsen needs to use the ring of lightning in the secret room that is opened with the Pyramid Shaped Key in the sewers of Citadel Island to break the sphere of ice and get the Sendell's Ball.
"Be careful, the devastating power of this ring consumes a great deal of Magic."