Collect 5 canopic jars to unlock the Onyx Pistols
Canopic Jars were used to store the organs of the dead. You are looking for jars used to store the stomach, with lids in the shape of the jackal-headed Duamutef, son of Horus.
You will find a canopic jar in the location someone on your friends list died. If no one on your friends list owns this game, or you are not sure where they might of died, you can create a second Gamertag and add it to your friends list. You can then load up the game with that Gamertag and die repeatedly somewhere close by.
Canopic jar rewards
5 (5+0) = Onyx Pistols
15 (10+5) = Onyx Submachine Gun
30 (15+15) = Onyx Assault Rifle
50 (20+30) = Onyx Marksman Rifle
75 (25+50) = Onyx Shotgun
105 (30+75) = Onyx Rocket Launcher