"It’s a two phase event. The first phase is open for typically one day. During the first phase the Treasure Hunt is open and you’re getting chests from destroyed monuments on enemy bases. Chests contain different event-specific awards such as energy packs, inner fire, etc.
The second phase is a battle one and open for typically four to five days. At this time the chests do or do not (varies from event to event) stop droping from destroyed monuments; regardless of it, the chests are now awarded to a player for successfully attacking a fortress in the event.