The Portal is an exciting game mode found by tapping on the "Hawkins" icon at the bottom left of the screen.
You can choose a path through the Portal, playing missions and collecting rewards as you traverse a simple maze. Inside the Portal, damage and energy of the your characters is carried over between missions, and a knocked-out character can no longer be used in the remainder of the Portal for that day. You can also collect artifacts that will give them strengths and abilities to help them reach the end.
The Portal resets daily, so be sure to check back every day to see what new missions await!