One of the first NPC questlines the player might happen upon is that of Yoel of Londor. Through this questline, the player obtains five free levels, five accompanying Dark Sigils, and up to 99 Hollowing levels. Mechanics aside, what role does Hollowing play in the Dark Souls III lore?
The item description for the Hollow Gem gives us a hint, as it is abundant in Londor, Land of the Hollowed. Combine that bit of knowledge with what we see in the into, and it's pretty clear being Hollow is the norm in Londor, where Yoel and his fellow pilgrims come from. After we find the Old Wolf Bonfire in Farrons Keep and work our way past the Stray Demon, we are again treated to a vista of the spot where we initially found Yoel. From this point it's clear the pilgrims sought to enter Lothric, but met a tragic, slow death at the dilapidated entrance. Yoel seems to be the sole survivor of his group, and is perhaps the only Londor Undead that fulfills his pilgrimage by finding an Unkindled Ash (you) to grant the Dark Sigil, and thus become the Dark Lord.
Yoel dies in the proces, and is replaced by Yuria as soon as you gained the fifth free level. In order to get the darkest ending, we must gain three more Dark Sigils. When we speak with Yuria after we talked to Anri and Horace at the bonfire on the Road of Sacrifice, she hints that we must eventually marry Anri. For what lord taketh no spouse? Once I reconnect with Anri we'll see where this questline continues.
It's description also notes it cannot be truly healed, but that a Firekeeper once brought great comfort to an Undead suffering the Dark Sigil. Mechanically, this is a hint that you can give the Firekeeper Soul to the Firekeeper in order to remove all Dark Sigils from your inventory. Doing this ends the Hollowing questline and prevents the player from accumulating more Hollowing.
Once you have obtained the five free level ups from Yoel, he dies and Yuria of Londor takes his place. Once you have met Anri her dialogue will indicate you will be paired in wedlock with this individual. If you do this, you obtain the final 3 Dark Sigils for a grand total of 8, which you will require to see the dark ending to Dark Souls III.
The wedding ceremony with Anri is actually quite horrific as the player will obtain a ceremonial sword and actually kills Anri with it. His sword can be found in the same location once that area reloads, indicating that that character has essentially left the game.