Ultra Dark Killer creates Ultraman Zero Darkness and makes him attack Ultraman Zero & Ultrawoman Grigio. They both lose their remaining energy in their fight. On the other hand Ultraman Rosso & Ultraman Blu struggle to fight with Ultra Dark Killer
Meanwhile the Blue Giant who is enjoying the sight of them inside the Dark Smoke gets found by Ultraman Ribut. He reveals the true identity of the Blue Giant to be Ultraman Tregear.
Ok! Boomer. We know that already.
So as the energy of Ultraman Zero decreases, all five Ultraman who were battling their Darkness, Dark Rugiel & Etelgar, transform into their Zero based forms.
The last one might not be a Zero form. But they have gained this form because of Zero's Training.
Damn! If you think about it now, Zero had crossover with each and every New Generation Heroes now.