I'll start the first rule of the Destiny drinking game. Let's see what else everybody comes up with.
Rule #1 Every time someone says gjallerhorn you have to take a shot (everybody will be drunk)
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PvE Everytime the Oversoul appears, take a shot, then kill the Oversoul. Everytime you don't get the first chest & have to wipe, take a shot. PvP Everytime you hear "Heavy Ammo Inbound" take a shot. Everytime you get stuck by a grenade, take a shot.