The Portal was a portal that has access to an alternate universe of the game we play, A Bizarre Day. It was located in the Fountain of The Peaceful Park.
The portal was a black small sphere, that was assumed to be a black hole at the start, but later, thanks to the power of the Fallen Star, someone, known as the "Hidden Hero" opened it. The Portal was then later shut down by a leader of unknown entities, possibly those behind the kidnapping of Tom.
Upon travelling through the portal, the player ends up in another universe, known as the Alternate Universe. This Alternate Universe was similar to the Original Universe we are familiar with. Not only the player, but a Potwagon Foundation Member also entered the universe, afterwards, he communicates with the player, telling them that we are in an Alternate Universe.
There was also another Portal in the alternate universe, it was assumed that the Potwagon Foundation member returned in time before it closed.
Initially, the Portal was like a tiny black hole, no features, just a black sphere. After it was opened, the Portal was a black, flat ellipse, this time, with the feature of teleporting to an Alternative Universe.