Fia is an interesting character that you first discover in Roundtable Hold. She has her own room behind the blacksmith and if you speak to her she will ask you if you wish to be held. This will give you a boon that will help you in battle. Despite her original appearance, this NPC has a very deep questline that will take you to the vast lands in between. How to complete Fia’s quest line in Elden Ring.
Start Fia’s questline by getting both D and Rogier to Roundtable Hold. They will each appear in the area after defeating Godrick. D can also be found in the northeast part of Limgrave outside of Summonwater Village. Before you can officially begin Fia’s quest line, you’ll also need to talk to her a few times and get her a hug. You also need to defeat at least one Sharbearer like Godrick and ride the Grand Lift of Dectus in Liurnia. Once that’s all done, Fia’s questline can officially begin.
After getting a Greater Rune and riding Dectus’ Grand Lift, return to the Roundtable Hold and talk to Fia. After a short dialogue, she will give you a Weathered Dagger. Bring the dagger to D and he will tell you he knows who owns it. Reload the area and pass into the room where the blacksmith is. You hear dialogue before entering. Go inside and you’ll see Fia standing over D’s corpse.
Enter the depths of the deep
After D is killed, Fia will tell you to go to the top of Deeproot Depths. For this you have to go to Nokron and find the Aqueduct dungeon. You can talk to D’s brother outside the boss room and give him D’s armor. This will unlock a special part of Fia’s questline. Defeat the pair of Valiant Gargoyles in the dungeon. At the base of the waterfall you will find a coffin. Rest in the coffin to be transported to the Deeproot Depths.
Defeat more bosses
Once in Deeproot Depths, head over the roots and into the ruins on the other side. Use the tree roots in the ruins to reach the area at the top of the cliffs behind the broken gargoyles. Enter the huge boss arena and go to the other side. A boss fight follows in which you must defeat five NPC enemies. After the fight, reload the area and talk to Fia, who is now sitting on a bench next to where the boss fight took place.
Obtain the curse mark and defeat the dragon
If you talk to Fia in the boss room, she will tell you that she wants you to get the Curse of Death. This is achieved by completing part of Ranni’s quest line. You can find it at the top of the Liurnia Divine Tower. Give Fia the Curse Sign and reload the area. Fia will sleep now. Interact with her to appear in another version of the boss arena and fight Lichdragon Fortissax. After the fight, talk to Fia and she will give you the Death Prince’s Mending Rune.
While the above is technically the end of Fia’s questline, you can take it a little further. Once you’ve given D’s brother the twin armor, reload the area and D’s brother will stand over Fia’s corpse. You can now get Fia’s armor set. Talk to D’s brother and he will be happy. Kill him to reset the twin armor and get the Inseparable Sword.
https://www.gamepur.com/guides/how-to-complete-fias-questline-in-elden-ring-all-steps How to Complete Fia’s Questline in Elden Ring – All Steps