system
Given the following post, respond as if you're commenting on Reddit:
human
[Dark Souls 3] What exactly are the Lords of Cinder?

Are they just powerful Lords, or are they individuals who previously linked the fire? Why are they needed to aid in linking the fire in this cycle? And are they associated in any way with the final boss?
gpt
Huge numbers of unmarked spoilers.  There's no way to answer this question without spoilers, so consider yourself warned.

The Lords of Cinder are all of the individuals throughout history who have linked the fire.  Whenever the First Flame comes close to dying out, time and space in the world immediately around it begin to warp in funny ways, allowing an immense number of "chosen ones" to simultaneously attempt to become the *true* chosen one who will link the fire.

As the number of cycles increase, "time" begins to break down, and the lords from previous cycles are appearing in this one as potential chosen ones.  However, all but one of the potentials is either too insane to come to the shrine, or has actively decided not to link the fire on account of how awful an experience it is.

The other kingdoms (Lordran, Drangleic) ended up becomming a nexus of hopefuls and past bearers of important souls from all across time and space.  In DSII, Undead separated by thousands of years of life experience would nevertheless come through Things Betwixt to arrive at the "same" place.  Lorthric is the kingdom at the center of this cycle, and it has suffered a peculiar fate from being so "late" in the cycle.  Not only are individuals drawn together, *entire kingdoms* have been mashed together into a single area of space, along with their peoples, and their lords.

The unkindled one needs the souls of the Lords for the same reason the previous champions needed the Lords Souls.  Their soul isn't strong enough to link the fire on its own.

The final boss, the Soul of Cinder, is an amalgamation of every previous chosen undead who has ever linked the fire.  The second phase of the final boss is Gwyn taking control, the first and still strongest soul contained in the cinders.