Ninjago, Season 7: Hands of Time

[The ninja come across by looking at a giant portrait of Young Garmadon and Wu fighting together in a battle]

Lloyd:
Look at them, my father and my uncle, Ninjago's two greatest Elemental Masters. I wonder if I'll ever live up to their legacy.

Jay:
First Kai and Nya, now Lloyd. It's family picture day at the museum.

Cole:
Does anyone recognize this battle?

Zane:
P.I.X.A.L., analyze.

P.I.X.A.L.:
Facial contour mapping indicates this scene takes place 40 years ago during the Serpentine Wars.

Cole:
During the Serpentine Wars? But Ninjago was at peace. Who would they be fighting?