Skyburners is a Grimoire card from the original Destiny (Destiny 1), found under Enemies → Cabal. Grimoire cards held most of the game's lore and were read on Bungie.net and in the companion app, not in the game itself.
Card text
Rumors of a yet-to-be-deployed Cabal Fleet have long haunted the Vanguard Hall. Hidden efforts unveiled the Phobos fleetbases house what is merely the first wave of a full-scale celestial demolition fleet, codenamed Skyburners. Ghost scans of the ships and reports of a large Psion presence have Warlocks anxious to learn of their weaponry. Dead Orbit assures the City that the Skyburner fleet poses no immediate threat to the City.
Grimoire text written by Bungie, shown as it appears in the Destiny 1 game files.