The Key of the Three Moons is a magical item that had been split into three separate mooncrystals stashed in An'telas, An'daroth and An'owyn, heavily-guarded hidden locations throughout Quel'Thalas, each constructed on an intersection of ley lines, similar to the Sunwell itself.
Each mooncrystal had a different color, and seems to be linked with different aspects of Ban'dinoriel. The first mooncrystal, cut from the emerald Eye of Jennala, opens the mind of the Gate Keeper, the second, cut from the amethyst Stone of Hannalee, opens its heart, and the third and last, cut from the sapphire Body of Enulaia, opens its soul.
Prince Kael'thas later recovered the crystals and, splitting the three between himself, Rommath, and Astalor, used them to overcharge and destroy the Sunwell, which had been irrecoverably tainted by Kel'Thuzad's resurrection. The crystals endured the Sunwell's destruction and had retained some of their power, but had also been corrupted by the explosion. Kael kept them in his possession from that day onwards known as Verdant Spheres.