Bungie has released a bit of information about what Destiny players can expect in December, with the main addition coming to the King’s Fall raid. This raid will get a Challenge mode and each week one of the bosses from the Raid will have a challenge, which will revolve around killing them in a specific way. In what manner the boss has to be dispatched will be revealed during that time.
You’ll know which boss is the challenge as there will be a skull sign in the Director, similar to the Nightfall events. The date hasn’t been given for when the first challenge will be issued, but the first week is the most likely time the bosses get tougher.
Source: Bungie