Keystone

The Keystone is one of five Bloodstones through which the Gods of Tyria forever shattered magic in the world of Tyria over 1000 years ago. The Keystone is the piece required to assemble the other four.

There is a theory conjecturing that the Keystone is the Scepter of Orr, as the scepter also has a bizarre power over the dead and has been round for generations. This theory cites the fact that the scepter merges with the bloodstone at Abaddon's Mouth, as the keystone was said to merge with the other stones to unlock their power.

For more information and background, read the Bloodstone article.