If you don’t want Fia to die, you should not give the Twinned Armor set to D’s brother. If you do that, next time you return, you’ll find him standing over her dead body. You can kill D’s brother to get the armor back. D’s and Fia’s quests are intertwined.
If you decide to keep the Twinned Armor set, go to Fia and let her hold you in her arms. Complete her quest to unlock the Lichdragon Fortissax
secret boss fight. To get there, give the broken dagger to D, go to Nokron and defeat the Valiant Gargoyle boss. Interact with the coffin located in the boss room to teleport to a secret Deeproot Depths location.
Find the Lichdragon Fortissax boss, and defeat it. You’ll earn 90,000 Runes, the Remembrance of Lichdragon, and the Mending Rune of the Death-Prince. You can use this special Rune to unlock the Age of Duskborn ending.
Let’s see what happens if you choose to hand over the Twinned Armor set to D’s brother.
What happens if I give the Twinned Armor set?
If you give the Twinned Armor set to D’s twin in the Siofra Aqueduct, D’s brother will kill Fia. This will lock you out of the the Lichdragon Fortissax
secret boss fight. You can still complete Fia’s ending even if D’s brother kills her. You get her mending Rune of Death before he kills her anyway.
To get D’s sword and armor set back, rest at a Site of Grance, reload and return. D’s brother will have drop the Twinned armor and the Inseparable Greatsword and you can pick them up.
⇒ Quick Tip: Don’t kill Fia early in the game, otherwise you’ll lock yourself out of her questline.
Is the Twinned Armor set good?
The Twinned Armor Set is one of the strongest early armor sets that you can get in Elden Ring. You can use it to get overpowered early in the game. You can wear it over or around your character’s torso to increase defense and resistance against enemy hits.
And now you know.