Shandalar's Wardstone

Shandalar's Wardstone is given to you by Shandalar when you are sent to Ice Island to retrieve his cloak. The wardstone is the raison d'être for the mages on the island to attack the party. They all believe, quite correctly, that it will enable them to get off the island.

In-game description
"This wardstone is a magical component to a "recall" type spell. When the possessor of this stone also has Shandalar's cloak, the stone will activate and transport the possessor and his companions back to the mage Shandalar. This stone's magics will only work in open air, not underground."