S1·E44 The Return of the Ancients

The descendants/survivors of a highly-developed race that was wiped out on Quasar return to the planet after a 1,000-year mission, and they're not happy to find out their civilization is gone. They attack and capture Dorno, and Gleep and Gloop bring the others. When Zandor confronts them they capture him and put him into a battle with energy whips against their leader. Tara and the Herculoids find proof of how the aliens' civilization was wiped out by trumpeter plants while Zandor plays for time. Tara gets the flower to Zandor which he uses to drive the aliens off.