Halo: The Master Chief Collection - Halo 2 - Regrettable Turn of Events - Halo 2: Collect the Prophet of Regret toy in Regret. This achievement is fairly easy to unlock.
On the mission Regret (Have the sputnik skull on), At the start of the mission exit and turn to the right where there is a destroyed bridge.
Perform a grenade jump over the gap. Then do another grenade jump to get on the level above. Walk around to the right where the toy is located.
Run over and it up and your achievement should pop! Enjoy!