The Rune Stone is magical stone that is linked to Queen Maeve from a deal with Mider. It is the source of Maeve's power, and link to the deep, dark pits. It is capable of teleportation, conjugation of creatures and various spells and illusions. When using the stone to conjure a creature, Maeve would utter, “Forces of sorcery, through the power of this stone,”, and then calls forth her desired monster.
At the end of the series, Queen Maeve uses all of its power at once to transform herself into a Naga-esque creature.
When Maeve was defeated, the Rune Stone vanished, leaving her without a way to use her magical powers.
Rune Stone's power in action.