The Isle of Prespur, is not often visited. It has only one possible harbor, and no real cover. It was originally the private estate of a mage of great power named Prespur, and on it he built a castle. Prespur of Netheril, a Netheril mage, studied and researched magic there in seclusion. It was his retreat when magic went afoul in Netheril, and his place of safety when Netheril fell. With his death, the island remained uninhabited for many years.
The terrain of Prespur is a combination of rocky beaches on it’s coastal areas with verdant greenland and rocky cliffs as one moves further inland.
The smaller island, Traitor's Isle, was unoccupied. The only site of interest is the Tower of Stars, created and used by Jorunhast, former Royal Wizard of Cormyr. The island's coastline was jagged and rocky, making travel here by boat extremely difficult and hazardous.
Eventually pirates landed on the island and investigated the castle, still in good repair despite the passage of time. They looted the valuables and started cutting the forest to make ships for their fleet. Then, for reasons undisclosed, they fled the island and spread tales of demons and ghosts. Adventurers who visited the islands through the centuries never found these demons or ghosts, but never stayed either. Pirates stopped there if necessary, but tended to avoid the place. The old castle started to fall into disrepair, and it was soon swallowed by the forest. The castle remains to this day, but it has suffered over the centuries. Moaning winds and leaky walls attest to the ravages of time.
A few years ago, an unseelie nymph moved to the island and began to destroy the natural life there. The forest died, and the island became like a desert. The castle once again could be seen from the waters, and the sight of it attracted explorers. These explorers unearthed the cellars of the ruins, previously undiscovered, and found the laboratories there. They also found Prespur's portal. The unseelie nymph kept out of sight and observed the explorers, stealing life from them if they stayed too long.
Discovery of the portal brought sages and wizards to the island to investigate, and eventually something killed the unseelie nymph or she abandoned the island; no one is sure which. In truth, she has not left, but she has found an underground lair in a cave on the far side of the island that she has hidden from view. She slowly saps the life of the investigators still.
The portal is a two-way, continuous portal that emerges on the island nation of Tharsult in the Shining Sea. It requires a key to reach Tharsult; those entering without the key are transported instead to the wastes of Icewind Dale (a one-way trip, as there is no return portal). The key for Tharsult is the word "travel" spoken in the ancient Netherese language. A simple doorframe on one of the basement walls frames the portal. The door set in the frame opens to reveal only wall behind; it is a false door. However, with the door open one can step into the portal.
On the Tharsult side, the portal is located near the sea, and it was once surrounded by a natural rock lip that formed part of a small ridge over the beach. Time and accident of nature have worn away the rock lip, and one can find the portal site now only by use of magic. Thus, it has not been found in all the centuries. The people of Tharsult live mostly in the port city of Urbeth, but they farm and hunt across most of their island home. The inhabitants discovered the portal site when mages began appearing in the same place on the beach about 10 miles south of Urbeth. The portal has not changed their lives much at all, as it goes to a basement of a castle on a nearly abandoned island.
Interest in the portal on the Isle of Prespur is currently limited to a few wizards who would like to control a fast way to the Shining Sea. After all, more trade travels on the Shining Sea than most other seas of Faerûn, and Tharsult is a large trading center. The chance for wealth is great, and the wizards investigating the portal have kept quiet about what they know.