When you find yourself ready with the fingerslayer blade, you'll need to be careful about the timing since giving it to ranni triggers some pretty significant changes. First thing to remember is dealing with preceptor seluvis - his questline completely shuts down once you hand over the blade, so get that magic scorpion charm from him while you still can. The timing here is crucial, you'll want to complete his amber starlight request and grab any puppet ashes you're interested in before proceeding.

Once you give ranni the blade, she'll reward you with the carian inverted statue which lets you access the divine tower of liurnia through the study hall but she'll disappear right after. The statue basically works as a key that flips the entire study hall upside down, creating a new path you'll need to navigate carefully. Make sure to position yourself properly when using the statue since the gravity changes can be a bit disorienting at first.

Before making the handoff though, take some time to thoroughly explore nokron especially around the night's sacred ground. There's valuable stuff like the mimic tear ashes after that boss fight, and you might want to stock up on upgrade materials while you're down there since you'll be heading into some tough areas soon. After giving ranni the blade you'll need to follow her quest through the ainsel river and eventually to the cathedral of manus celes, so having upgraded equipment will definitely help with what's coming.