A special attack that uses the combined energy of everyone in a cast with the coup-de-grace, usually but not neccessarily, being delivered by the lead. It can be an actual attack technique, or it can be used as a powerup for something, such as an attack.
A common subtext to the trope is that the worst villains are inevitably weaker because they don't have friends to rely on (see The Power of [...] [tvtropes.org]