Sky Stone

A powerful artifact guarded by the Sky Sentinels, the Sky Stone not only powers the antigravity engines and defenses of the Skytower but also communicates (often frustratingly vague) instructions and insights.
Over the years, more scientifically minded members of the Sky Guardians have studied the Sky Stone, but have only succeeded in ruling out potential explanations for the stone's origins. It shares some characteristics with known Cosmic Crystals (great power, semi-sentience) but is significantly larger and does not appear to be able to fuse with a living being. Likewise it seems unlikely to be an unusual manifestation of the Prism Force.