Skulltaker Halo 2: Assassins
From Halopedia, the Halo wiki
Skulltaker Halo 2: Assassins is an achievement in Halo: The Master Chief Collection. It is awarded for collecting the Assassins skull in Halo 2: Anniversary on Regret while playing on Legendary difficulty. The achievement is worth 5 Gamerscore, and the Xbox achievement art is a still of ODST Walpole aiming his SRS99-AM sniper rifle from the Halo 2: Anniversary version of the bonus cutscene Another Day at the Beach.
Immediately after the gondola ride following the battle with the Hunters, there will be a building with two turrets on it. Enter the building and turn to your right. There will be a box, jump on it. From there jump to the adjacent light fixture, then to the ledge. The ledge leads to the area with the two turrets.
Once there, Grenade Jump to get onto the roof area. Then, using Crouch Jumps and/or Grenade Jumps, follow the right side perimeter of the building. Once you have gone as far as you can go you will find a small grassy area with two Stealth Elites holding Energy Swords in their "angry stance" and the Assassins Skull between them.
As in the original game, the skull itself appears on all difficulties, however it cannot be collected nor can the achievement be unlocked on any difficulty save for Legendary.
Note: sometimes, the achievement can be unlocked by merely exploring the upper levels of the structure before ever reaching the skull.