The Demon Gates is an exploration bounty in Act III of Diablo III, found in Arreat Crater Level 2.
The player must destroy 4 Demon Gate objects scattered throughout the middle portion of the map. They look the same as those encountered during the fight against Azmodan - large spiked totems hovering over demonic portals in the ground - and both are similar to the Demon Summoners sometimes found in Arreat Crater in Campaign Mode. Upon approach, each gate will rapidly emit several orbs of demonic energy which turn into Demon Troopers, and several Demonic Hellflyers may swoop in as well. This variety of Demon Gate is destroyed by channeling on it without being interrupted. Clearing the spawned monsters is not strictly necessary, but likely the easiest way to complete the bounty quickly.
When the last Demon Gate is destroyed, a Radiant Chest will appear.