Direct answer: Ainsel River has two halves and only one of them is open to you early. The southern part is entered freely by the Ainsel River Well in Liurnia. The northern part, Ainsel River Main, is locked until you have given Ranni the Fingerslayer Blade. That lock is the single thing that makes this region confusing.
The southern half
The Ainsel River Well is free — no key, no quest. Take the lift down and you get a large cave system of open caverns, ruins, and waterfalls. It is less disorienting than Siofra and much smaller.
Activate the Ainsel River Sluice Gate grace when you arrive. The confirmed Sites of Grace here are Ainsel River Well, Ainsel River Sluice Gate, Lake of Rot Cliff, and Frenzied Flame Village.
This half contains the Lake of Rot and the Cave of Knowledge, a Dragon Communion site. The Lake of Rot leads to Astel, Naturalborn of the Void — but see the gate below, because you cannot actually finish that fight’s follow-through without the Dark Moon Ring.
Unlocking the northern half
You need the Fingerslayer Blade, which means Starscourge Radahn defeated and the Blade retrieved from Nokron in Siofra. Hand it to Ranni in her Rise, and she opens the way.
There are then two routes in, and either works:
- Renna’s Rise waygate — the intended route. From Ranni’s Chamber, go down the stairs, ride north-east to Renna’s Rise, climb the ladder, go upstairs, and use the waygate.
- The coffin in Deeproot Depths — the alternative, and the one that ties this region to the underground wave.
Both land you in Ainsel River Main. Whichever you take, activate the grace on arrival before exploring.
What is actually up there
Immediately in front of you: Ranni’s miniature doll. Talk to it three times from the nearby grace and Ranni enlists you to kill the Baleful Shadows deeper in, past Nokstella. Killing one gets you the Discarded Palace Key.
That key opens a chest in the Grand Library of Raya Lucaria containing the Dark Moon Ring — which is the item that unlocks the rest of the Lake of Rot, including the Moonlight Altar. The ring is the keystone of Ranni’s entire questline.
Evidence status
Research-reviewed. The free Ainsel River Well entrance, the two unlocks for Ainsel River Main, the Renna’s Rise waygate route, the confirmed Sites of Grace, the Discarded Palace Key, and the Dark Moon Ring’s location in the Raya Lucaria Grand Library were each cross-checked against multiple independent references. Not gameplay-verified on camera.