The Portal to the Last Millennium, as the name implies, can be used to teleport to the Last Millennium dimension.
The Last Millennium dimension is an infinite void with nothing in it, aside from a structure that spawns where the Portal takes you.
Each Portal is "owned" by the player that placed it. Using the Portal will teleport you to that person's location in the dimension. Even if they place more than one Portal, they always take you to the same destination.
As more players make their own Portals and get their own "home locations" in the dimension, those locations are created in a spiral with 1,024 chunks between them, by default.
