Direct answer: the Fingerslayer Blade is in Night’s Sacred Ground, in the upper part of Siofra River, inside the chair-crypt holding a vast corpse. Getting there requires you to have already beaten Starscourge Radahn — Nokron is locked until then. This is the single most important item in Siofra for anyone running Ranni’s questline.
Why Radahn is the gate
Nokron, Eternal City is initially inaccessible. The route opens when a star falls upon Limgrave, which is the in-game event marking Radahn’s defeat. Until then you can stand at the entrance and it will not open.
This trips people up because Siofra itself is available from the very beginning of the game. You can be standing in the region and completely unable to reach its most important content, with nothing on screen telling you why.
The route once you have Radahn down
From Siofra, make your way north into the towering ruins of Nokron. The Sites of Grace up here are Night’s Sacred Ground and Nokron, Eternal City, and there is also a Mimic Tear grace nearby.
In Night’s Sacred Ground, the Blade sits under a chair-crypt that holds a vast corpse. It is described as a Nokron treasure said to be capable of harming the Greater Will and its vassal Fingers — which is why it is the whole point of the questline and not just a weapon.
The Regal Ancestor Spirit is also up here, in the northern Hallowhorn Grounds north-east of Night’s Sacred Ground, and needs only six flames lit rather than eight. It is a much harder fight than the southern one — at half health it heals itself by draining both you and the spirit animals in the arena, sometimes collapses and re-emerges from a different animal, and borrows the movements of a rabbit, a boar, and a goat. Worth knowing before you commit.
What the Blade does for you
It is the prerequisite for giving Ranni the Fingerslayer Blade, which is the decisive step in her questline and the requirement for her ending. The reward is the Dark Moon Ring, described as a ring of darkness bequeathed by the moon goddess, offering magical protection and enhanced intelligence — and it is one of the most commonly sought talismans in the game.
There is a practical side effect worth knowing: the northern Hallowhorn Grounds is also a convenient source of the Winged Greathorn via the Remembrance of the Regal Ancestor. It is honestly a mediocre weapon, but it is a spear with a self-buffing skill and that has some appeal to a spear build.
Evidence status
Research-reviewed. The Radahn gate on Nokron, the Fingerslayer Blade’s location under the chair-crypt in Night’s Sacred Ground, the six-flame requirement for the Regal Ancestor Spirit, its healing and animal-possession mechanics, and the Dark Moon Ring reward were each cross-checked against multiple independent references. Not gameplay-verified on camera.