Every other Thursday, Dan from maintenance appears in a small vending kiosk at the very end of Cortana, replacing the Arbiter (only on Legendary-Yolo). He proceeds to kill everything with his Faze-Ass Spartan Fazer which he'll then trade with you for a plasma pistol and some dewritos. Then you'll unlock the Spartan Fazer with unlimited ammo (powered by Mountain Dew) which can be dual-wielded with every gun in the game. Thank me later...