Luna is unconvinced of Mark Wells' capabilities and challenges him to prove himself. One massive lightning bolt later, Luna stands in stunned shock on grass burned where the electricity grounded out. Celestia asks if Luna is okay and her sister replies, "I may have underestimated him." Celestia loses it.

This is a scene from our story, "Off The Mark", after Celestia and Luna finally come back to Equestria.
https://www.fimfiction.net/story/41.....5/off-the-mark
Art by  foxenawolf