Skulltaker Halo 2: Grunt Birthday Party
From Halopedia, the Halo wiki
Skulltaker Halo 2: Grunt Birthday Party is an achievement in Halo: The Master Chief Collection. It is awarded for collecting the Grunt Birthday Party skull in Halo 2: Anniversary on The Arbiter while playing on Legendary difficulty. The achievement is worth 5 Gamerscore, and the Xbox achievement art is a still from the Halo 2: Anniversary cutscene for Delta Halo, depicting a Unggoy Minor on a Eeo'Pimu-pattern Shade.
The very first time you get a Banshee, fly down. Below you is a large beam-like structure (not to be confused with the one directly below you that turns and goes vertical) running from the arm of the station you just came out of to one exactly like it on the side opposite. Below this structure are three cylinders hanging down, with cylinders of light shooting out of the bottom(and the sides). They look like they are what keep the station afloat. If this explanation is too vague, they are the absolute lowest parts of the station. Now go to the middle engine and land on the top part of it. This and the other engines connect to the large structure mentioned earlier. The tops of the engines come up to meet the structure at an angle, where this angle is under the connecting structure and coming up to meet it, there is a place to walk (like being under a bridge). Under one side you will see the six Heretic Grunts dancing in a circle around the Skull.