The Smash Portal is a new item introduced in Super Smash Bros. Millennium, which has been nicknamed the "Last Resort" by the fan base of the franchise. The item, once grabbed, creates a portal on the stage that any character can enter. Once entered, it transports the remaining fighters on the stage to a different stage, allowing for more dynamic game play. Some stages have pre-built portals into them, which can be entered at any given moment. They can also be activated by fighters when they're launched off the stage through a specific input command, but this has a very unlikely chance, with only 4% accuracy.
It also returns in Super Smash Bros. Carnival, but it is unknown what definitive effects it has.