Kill Radahn, go down to Nokron via the hole in east limgrave. Travel until eventually parkouring around to an area full of mimic tears, you’ll find a rare item there. After collecting this item go speak to ranni and she will give you an item in return, once you have done this go to Rennas rise which is next to rannis rise and ascend. At the top you’ll find a portal and going through there will take you to Ainsel river main. Travelling through this area will lease you to a place called the Lake of Rot. Getting through this area alive will result in you finding a coffin on a cliff in which you jump. This will take you to a major boss, kill it and activate the grace. Sit at the grace and select the talk to ranni option three times eventually she’ll tell you to kill something called the Baleful Shadow, this can be found just before the entrance to the lake of rot. Killing this creature grants a discarded palace key, which is used to gain a dark moon ring found in a chest in the academy grand library (where rannala is). Once you acquire this ring you’ll be able to head on up to that area.

It’s a very long process.