This page of the Elden Ring game guide contains information on where you can meet Fia. You will also learn how to complete her series of tasks and what rewards await you after completing them.
You will meet Fia for the first time in the Roundtable Hold. She is in a room with a mirror that changes your character's appearance. You can get to the Hold after defeating Margita or after exploring other regions.
How to proceed with the quest?
Aside from hugging Fia, there's not much you can do to move her quest further. You can learn what to do next after you unlock the first Site of Grace in the Altus Plateau region.
After you've unlocked the Site of Grace in the Plateau, head back to the Roundtable Hold and hug Fia. Then choose the option that will allow you to hear a new dialogue. You will receive a key item, a dagger, which you must hand over to an NPC called D, Hunter of the Dead.
After resetting the location, a room in the Hold will unlock. It is in front of the blacksmith and Roderika. Go there, then talk to Fia. After a short conversation, Fia will disappear. You will be able to pick up D's bell, which will re-unlock the purchase of its incantation and his Twinned Set.
Then, you have to go to the coffin, which is located in the boss arena of the Valiant Gargoyle, located in Nokron, Eternal City. A cut-scene will then start. After it, you will find yourself in a new location, Deeproot Depths.
You will mainly fight giant ants, both on the ground and in the air. Fia is at the very end of the location. It's not too big, but you have to watch out for the numerous enemies.
Then, when you reach the very end of the location, you will have to fight Fia's Champions. It is a boss fight. The fight itself looks like the raids of other NPCs on your world.
After defeating the champions, Fia will sit near the Site of Grace and portal. Talk to her and then hug her if you want to move the quest.
Then you will receive a quest to find the Cursemark of Death. This item is related to Ranni's quest. After receiving the inverted statue from her, you must go to the Carian Study Hall in Liurnia of the Lakes.
At the entrance to the Carian Study Hall, you must place the inverted statue next to the globe. This will make the entire library turn 180 degrees. After using the elevator, you will get to the top of the Divine Tower of Liurnia. That's where you will find the Cursemark of Death.
After returning to the location where Fia is, you can now give her the Cursemark of Death. Then, after resetting the location again, Fia will fall asleep. By touching her, you will move to a location that will lead to a fight with a secret boss, Fortissax.
After defeating Fortissax, he will give you the Mending Rune of Death-Prince, which is required to unlock the alternate game ending.
You can end the quest here, gaining the opportunity to unlock a new ending, or go back to the Site of Grace called Great Waterfall Crest and go back to the place where the scared NPC was.
You can give him the Twinned Set that fell from D, Hunter of the Dead in the Roundtable Hold.
Spoiler alert! The frightened NPC is actually D's younger brother. If you hand him the Twinned Set, he will get to the Deeproot Depths and kill Fia, to avenge his older brother's death. However, the decision to hand over the set does not affect the fate of Fia.
Reward for completing the quest
The reward for completing the entire quest related to Fia is, among other things, unlocking an alternative ending to the game.
Moreover, only by completing Fia's quest you can fight Fortissax and get his remembrance.
In addition, you will also get Fia's outfit at the end, consisting of a garment and a hood.