Information on the story! :)

A long time ago when the First Spinjitsu Master was present.
A companion of his was there to protect him when he needed it the most and helped provide light to Ninjago, the place that the master created before disappearing. The guardian then decided that it was time to rest after serving its master.
So, the guardian proceeded to a temple to rest and said these words,
"The person who shall have my powers will be the guardian of the green ninja and its protectors. May the light guide you through your journey."
At that, the Guardian of Light finally rested with his master leaving Ninjago to advance on its own and to hope that future heroes will protect what the two left for the next generation.