So, after my last playthrough I think I finally figured out Londor. It's goal always seemed obvious (usurp the fire duh), but their modus operandi, and the role it's various agents play, not so much. So here it goes:
- Londor Modus Operandi:
1) A faith, sedimented in the Sable Church, proselityzing the non-kindling of fire and welcoming the age of dark. It seems to accept undead people, hollowing or not, to embrace it and become "pilgrims". The old pilgrim NPC overlooking the dreg heap in the DLC supports this, as she was a former handmaid from Lothric kingdom herself, according to her ashes description. Ergo, she was a normal undead at some point that "converted" and became a pilgrim.
2) Pilgrims sent across eras to hinder the linking of Fire. How this hindering is done is not entirely clear, but it seems to involve them seeking either A. firelink shrine, B. the kiln of first flame, or C. the current candidate to link the fire/Lord of Cinder (maybe the church sends them to all three). Then they must die nearby this target and "undo their shackles" as spoken by Yoel, supposedly letting their humanity burst through the chains/carapaces in their backs, and become "butterflies". Said butterflies then swarm the target, whatever it is, and try to stop the rekindling of the Fire (even hurting potential helpers if needed, like they do with the Ashen one).
3) A search for, and grooming of, a potential Dark Lord. A task apparently conducted more directly by the three sisters with help of personal retainers, like the pale shades, more prominent pilgrims (like the disguised one spying on Anri), and perhaps even Church hollow knights like Vilhelm in his time of service. Notice how Yoel finding the potential dark lord in undead settlement seems a coincidence, his task was to die and become a buttlerfly, not search the dark lord.
Obs 1: Londor culture and symbolism seems to come from the Ringed City, the fabled "City of Men" from the distant past. One can trace that city's "legacy" from DS1 New Londo and it's architecture, which also portrays the symbol of Avowal in it's towers, to the Morion Blade that has similar shapes, and the Sword of Avowal used in the ritual of transferring the curse from Anri to the Ashen one. Even the name "Londor" seems to come from New Londo. If Londor inhabitants are aware of these links, or if only Kaathe - the probable responsible for preserving said culture/symbology - is aware, is not clear.
Obs 2: they never seemed to have found the Ringed City, judging by the old pilgrim speech at the dreg heap. She refers to the city more like a fable than a real place. Which implies the Sable Church also don't know if it's real or just a fable, nor it's exact location. Ironic, assuming they carry it's legacy.
Obs 3: how the pilgrims travel through time is beyond me, but they most definitely do, as seen in the intro. It's possible Londor is not even in the same time/dimension as Lothric kingdom portrayed in the start of the game, but already at the "end of the world" shown in the intro / time of Gael fight. Some space-time travelling options shown in the game are A. the bonfires, but for this one needs to be unkindkled theoretically; and B. through "portals" like seen in Oceiros backyard which links "present" Fire-era Lothric to the "past", dark-era Lothric.