Knights of the Sunless Realms:
Sirris, and originally her grandfather Hodrick, before he went mad, were holy Knights of the Sunless Realms. Sirris has an armor set with a white feminine veil as well as draping veils over the shoulders, very similar to Yorshka's apparel.
Yorshka represents the Darkmoon Company, and bestows the Darkmoon Blade miracle to its top Knights. The player, after killing 30 PVP invaders as a Darkmoon Knight, is given the Darkmoon Blade miracle. Sirris also has the Darkmoon Blade miracle, indicating that she, and perhaps her grandfather before her, were the last Knight of Yorshka's company who she says disappeared. The Sunless set says "Sunless Knights serve the nameless moon" so I think that's about as solid as you can get as a canon relationship.
What's interesting is that in the Borreal Valley, the moon is always out, as per the item description of the doll. It almost seems as if Sirris' "Sunless Realms" could be the Borreal Valley but I'm inclined to believe they aren't so misleading as to give us two names for the same place. Perhaps the Borreal Valley is one of several places in the Sunless Realms.
Engraved with a brilliant flame crest that burns brightly within the order's customary sunset colors. This design symbolizes those who both harbor and oppose the power of flame.
So there are these holy Knights fighting against the power of flame. They envisage a world without sun, without fire. Yet there's nothing to lead you to believe they want the abyss, or an age of dark. The Sunset Talisman, which Sirris uses to cast miracles, mentions
Talisman given to the knights of the Sunless Realms, who serve the nameless moon.
Scales with intelligence, which is rare for a miracle catalyst, but makes it compatible with Dark. In the Sunless Realms, this fact is related first as an initial warning.
that while there is the byproduct of Dark being accessible through servings he moon, they are warned of this potentialit is not encouraged.
There could be a spell cast by Gwyndolin before Aldrich ate him that causes the sun to constantly be in a state of total eclipse, as evidenced in Irithyll. Pure speculation, that last bit. I just wonder why he sun is permanently eclipsing in Irithyll.