The Vex Invasions on the Moon have a chance to drop powerful gear. Shortly after public events finish, a Vex portal will appear nearby and a Vex Gate Lord with lots of minions will spawn. Kill him and another portal will open elsewhere, do the same and a 3rd portal will open, kill this one too. Now there is a random chance that a 4th portal will open and a big Overlord Hydra will appear, kill this and there is a Powerful Reward in the chest he drops. You can do this twice per week, per character.