The Flamekeepers effectively came to an end when the Flame was destroyed to save Madi Griffin] from Sheidheda who had originally been killed by his own Flamekeeper. The last Flamekeeper, Gaia, is left unsure of her purpose after this.
Flamekeepers are the leaders of the Grounders' faith which centers on the Flame. They search for Nightblood children, train them, and oversee the transition between Commanders. The head Flamekeeper serves as an advisor to the Commander of the Coalition. When there is no Commander, the Flamekeepers are responsible for keeping the Flame safe.
The Flamekeeper uses their scouts to find Nightbloods. When a Nightblood child is discovered, they are brought to Polis to begin training for the Conclave, which is the process in which the new Commander is decided.
When a Commander dies, the Flamekeeper removes the Flame from their neck and initiates the Conclave. The belief among Grounders is that the Flame, a.k.a. Commander's spirit (which is A.L.I.E. 2.0), "chooses" its successor through this process. Once the victory horn is sounded, the victor is taken to the temple where the Flamekeeper performs the ascension ceremony. The Flame is implanted into the neck of the victorious Nightblood making them the new Commander.