The Runestone was an ancient monolith erected by the Elven Druids and inscribed with powerful runes of protection and warding. The Runestone, seized by Gul'dan and his Ogres, was eventually hewn into slates that were then used to construct the Altars of Storms. The Elves, knowing that their sacred artifact had been defiled to create the Ogre-Magi, have sworn to destroy all of the unholy Altars across the kingdom.
The Runestone is a unique non-buildable building appearing in the Tides of Darkness mission The Runestone at Caer Darrow, where the player's objective is to capture it.
The Beyond the Dark Portal mission The Heart of Evil uses damaged - and therefore burning - Runestones to represent a part of Ner'zhul's Mystic Sanctum.
Two Runestones are present at the entrance of The Tomb of Sargeras in Beyond the Dark Portal. They have a purely decorative function in-game.
Even though it cannot be built, the Runestone is given a build time (175 seconds) and cost (900 gold, 500 lumber) in the game files.