Moonlight Greatsword AKA Dark Moon Greatsword
Getting this sword is definitely something you should do, especially if you are trying to get your Platinum trophy.
SPOILER ALERT
Everything you read below this comes with a big spoiler alert. Alright, brave soul, let’s carry on. One of the first things you’ll notice is that this sword has some frost added to it, in particular, it has frostbite and it is very potent in this game.
Step 1
In order to get this sword, you need to complete the full Ranni Quest line, and it is a very long one, so buckle up. To start the Quest, you want to start the Church of Ella. You go there after you have obtained Torrent from the Gatefront Shrine. You meet Malia and she gives you access. If you come back to the church during the evening, you should meet Ranni as she’ll be chilling and she will give you the Summoning Bell Key Item.
For the rest of the steps, as well as some spoilers, please refer to the video.