Oracles are a Vex unit found in the Vault of Glass. Unlike normal enemies, they do not attack directly, but they are still very dangerous.
There appears to be three types of Oracles; yellow, which appear during the fight with the Templar, red, which appear during the fight with Atheon, and green/Taken, which appear during the fight with Morphon, Blighted Mind. The yellow oracles cast a debuff called "Mark of Negation", which they cast on the entire party unless they are killed quickly. The debuff will kill anyone who still possess it when the Templar casts "Ritual of Negation." The red oracles will simply kill the party outright with an ability called "Prophecy of Doom" if not destroyed in time. However, killing them all will grant the fireteam a buff called "Time's Vengeance", doing bonus damage against Atheon.
Green/Taken Oracles are periodically summoned by Morphon, Blighted Mind, who will emit a signature warbling tune used by the Templar before creating one. Taken Oracles instead of serving as a time limit instead make Morphon immune to all forms of damage while they live and must be destroyed before Morphon can be damaged. Destroying them temporarily stuns Morphon.
