The Dauntless site has been updated with a new blog post that reveals the "shifting chaos" of the playable locations that come and go over time. At any given moment, there are three islands visible and playable on the map. One, Dyadic Drift, is a permanent resident and remains in place. The other two are "rotating islands" that are available for a specific period of time before leaving the rotation until an indistinct point in the future.
Each of these islands also features specific Behemoths to hunt. For instance, the Bloodfire Embermane is only available when Vermillion-18 is active on the map. Players hoping to hunt it will need to do so before the island's timer runs out over a 2-week period. The Behemoth won't be able to be hunted again until Vermillion-18 reappears on the map.
Check out more details by visiting the Dauntless site.