So the hero has spent the entire first installment fighting the main villains, and, now that the series has recieved a second installment, the hero is ready to go for round two with their archenemy... until a new, Bigger Bad shows up. The new villain wastes no time in humiliating/defeating the old villain and completely usurping their role as the Big Bad. And you know they mean business because they've just proven themselves to be more powerful than the villain that the heroes were fighting for the entire first installment.
The troper namer for this trope is Xemnas from Kingdom Hearts II. Xemnas pulled a Xemnas Complex over on Disney's Mistress of All Evil herself, Maleficent, through his Dragon, Saix, defeating her in Hollow Bastion and his mooks proving themselves to be superior to Maleficent's mooks.
This trope can be good or bad depending on how the writers use it. Typically, though, it doesn't go over particularly well with the fans because, more often than not, they're already attached to the original main villain. However, a competent writers can employ it to set up interesting character developments for both the original villain and the hero.