This is in the Bright Lord missions. As you complete the main missions, you will unlock secondary missions (in white on the map) to increase the ring's power. There are four of these, once you complete the last one, you will receive this achievement. Each mission is unlocked by reforging the nearby tower.
You MUST complete all secondary missions before you do the final main mission. You will need to start over once the Bright Lord is completed. Also, you'll probably want the extra power during the final mission.