The Portal set is not an expansion to the main Magic: The Gathering game.
It is instead a new game based on the same rules, but with many complex
issues removed from the game. It has a certain amount of compatibility
with the core Magic game, but it is not entirely the same.
Portal was printed with black borders on the face of the cards. Cards can
be identified by the expansion symbol on the right hand size of the middle
section. The expansion symbol is a circle with radial lines on it
is supposed to remind you of a tunnel or possibly a door.
The 222 card set's breakdown by rarity looks like this:
It was released in early June 1997.
The print run is unknown.