This is a Halo 3/Halo: Reach mixtage from Sandman, edited by SillyGoose entitled, "4/14", made to celebrate their birthdays. The gameplay consists of leftovers, but it's Sandman so they're still ridiculous. The editing is pretty good well, despite Sandman doing it at the last minute. The Sandman/SillyGoose duo has never disappointed me yet, and this montage certainly wasn't an exception. Enjoy!
YouTube - 4/14 - A Halo 3/Halo: Reach Montage