From Destinypedia, the Destiny wiki
Together with the Templar, Oracles guard the way into the deeper Vault. According to legends, Oracles foresee what is to come, a world as the Vex desire it - and the Templar has the power to shape reality to match the Oracles' design, expunging any threats.
Unlike normal enemies, they do not attack directly, but they are still very dangerous. They appear as glowing cubes of strange particle energy hovering stationary in place.
There appear to be three types of Oracles: yellow, which appear during the fight with the Templar; red, which appear during the fight with Atheon, Time's Conflux; and green, 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 Oracles are periodically summoned by Morphon, Blighted Mind, who will emit a signature warbling tune used by the Templar before creating one. These Oracles, instead of wiping Guardians after a certain time-limit, cast an invincible shield about Morphon, making it immune to all forms of damage; they must be destroyed before Morphon can be damaged. Destroying them temporarily stuns Morphon. This is repeated three times.
List of appearances