Successfully complete the last level of the game incampaign mode under the Legendary difficulty setting. The newending features a human and alien fighting hand to hand. The humanwill say "Its over", and then they hug because theyknow the explosion will kill them both. Also, the FMV sequenceof each battle scene will be slightly longer.
When you are in the Silent Cartographer room the firsttime, get in the Warthog and drive in fast. You can make it throughthe door without having to get to the security override. If youdo not make it, equip a pistol or assault rifle. Walk up to thelocked door. If you are centered enough, you should see the energysword person's head through a small pane of glass. Shoot him repeatedlyin the head and he will die.
Obtain the indicated skull to get the corresponding effect.
Simply beat the game on legendary to unlock an additional scene at the end cinema.
Towards the beginning of the game, there is a way to bypass the two weapon limit and carry three weapons at once. On the first level, make your way to the bridge. After speaking with the captain, quickly run out of the bridge before the chapter name disappears. If you were fast enough, the checkpoint at the door will not trigger. Now run to the mess hall and get your Assault Rifle and another weapon. Now, run back to the bridge and trigger the checkpoint. When you go back towards the mess hall, the pistol will appear in your hands, and you will be carrying three weapons instead of two. Each gun reloads and exchanges normally. Note: This only lasts for the first level.