The Portals were guardians of the Tkon Empire that confronted all those seeking entry to the Empire and decided which petitions for entry would be granted. Portals were invested with tremendous power, including the ability to immobilize starships and to manifest themselves in various corporeal forms according to the appearance of the petitioners.
Portal 63 was a guardian on a planet in the Delphi Ardu system that was active over 600,000 years ago during the Age of Bastu as time was marked by the Tkon. After that he remained inactive until 2364 when the crew of the USS Enterprise (NCC-1701-D) encountered his outpost. At that time the Portal learned that the Tkon Empire had fallen shortly after the Age of Makto and that he had been inactive all that time.
The Portal took an interest in the Federation and, in particular, William T. Riker who had descended to the planet to investigate the force field that Portal was using to hold the Enterprise. After putting Riker through a trial, Portal ultimately released the ship and returned to his post until he was needed again. (TNG: "The Last Outpost")