Fia's questline, the Deathbed Companion, is crucial for unlocking the 'Age of Duskborn' ending. It begins by accepting her embrace at the Roundtable Hold (Nightfall). You'll need to acquire the 'Cursemark of Death (Nightfall)' from the 'Divine Tower of Liurnia (Nightfall)' and the 'Hallowbrand (Nightfall)' from the 'Deeproot Depths (Nightfall)'. The final confrontation with the 'Lichdragon Fortissax (Nightfall)' is now even more challenging, as it wields dark lightning and 'Death Blight' more aggressively. Completing her quest will grant you the 'Mending Rune of the Death-Prince (Nightfall)', allowing for a unique ending. Be prepared for difficult boss fights and intricate lore.
