A Rune of Power was a magical artifact, usually a stone or other small object bearing an inscription. This inscription was a magical script which held some mystical power. Many runes were used as keys for various magical mechanisms.
Shilla, queen of the succubi, used a rune cage to entrap Arokh. The magic of the cage prevented Arokh from escaping and also from anyone trying to break his chains. Rynn had to take the rune key from Shilla to save him.
Drakan: Order of the Flame - Drakan: The Ancients' Gates