The Alliance Strikes Back! - S2.E19

All at once, Zarkon's countless slave planets begin revolting at the same time. The evil King's armies are spread thin, and the Alliance stages a major offensive. Princess Allura and the Voltron Force once again take the fight to Zarkon. Zarkon fights back by trying to sacrifice Prince Lotor, but Merla has a change of heart and protects him. Then she begs the Voltron Force to help her save Lotor from the Pit of Skulls.