Orpheus is an achievement in Halo 3 and Halo: The Master Chief Collection. It is unlocked by beating the par score on the level Cortana. The achievement is worth 10 Gamerpoints. It is represented by a blue circle with a fist and lightning bolts in it in the original game and a screenshot of High Ground in the collection.
The par score for is 15,000 points in Halo 3 and 17,000 points in The Master Chief Collection; the difference is due to the fact that the two games feature different campaign scoring formulae. Though the achievement can theoretically be unlocked on any difficulty, more points are awarded in campaign scoring when playing on higher difficulties.
In the original game, players must achieve the par score individually, even when playing cooperatively; in The Master Chief Collection cooperative players share a single score, potentially making the achievement much easier.
The name Orpheus is most likely a reference to the Greek myth of Orpheus, a man who entered the underworld to rescue his wife, just as Master Chief entered the "underworld" of the Flood-infested High Charity to rescue Cortana.
On Normal, activate the Tough Luck, Catch, Fog, and Famine Skulls because for the Tough Luck and Catch, they don't dodge or throw grenades. It would be a good idea to use a Shotgun/Energy Sword in combination with a Battle Rifle/Carbine. Also, try to get as many medals as you can -- for example, grenade sticks or headshots. Headshots can be obtained by firing a headshot-capable weapon, such as the Battle Rifle, at the feeder tendrils on a Flood Combat Form's chest; this kills the Pod infector controlling the body.
Note: When trying to get the campaign scoring achievements for any level containing the Flood, kill as many Combat Forms as possible with weapons that do not destroy the Combat Form's body. Once the area is clear, back up and let the Pod infectors resurrect the fallen Flood. Kill all the Combat Forms again to gain more points. If no Pod infectors remain and there are blister-like sacs on the wall near you, pop them by shooting or meleeing them to release a small amount of infectors.