If you look closely, the Cultists have another 517 hours of channeling before they finish their cast. (This screenshot was taken earlier this week.) That means in about three weeks, Fel Channeling will be finished. Will this actually do anything? Probably not, if history is any indication. However, it may signal the coming of a small patch.
Players have been speculating in the comment section of WoWhead since the site first reported this new channel. While there are plenty of suggestions as to what this countdown could be signaling, the most obvious event would be the quest that causes players to lose their artifact weapon.
The all-powerful, historic weapons introduced in Legion will not be coming along for the next expansion. Thanks to some datamining, we know the scenario where something happens to our weapons, rendering them useless by the time Battle for Azeroth comes around this fall.