D, Hunter of the dead and D's twin brother

Meet D either on the east side of StormhillLandmark or in Roundtable HoldLandmark.

Acquire a Deathroot (defeating the Tibia Mariner in Summonwater VillageLandmark is the quickest way) and show it to D. He will mark a teleporter to the Bestial SanctumGrace on your map

Take the teleporter and speak to Gurranq at the Bestial SanctumGrace. Return to D and select yes in his dialogue.

Deliver Fia's dagger to D. Next time you enter Roundtable HoldLandmark, D will be dead. Take his Bell Bearing and the Twinned armor set.

Find D's twin brother outside the Valiant Gargoyle arena in Nokron and give him D's armor.

After defeating Lichdragon Fortissax and completing Fia's questline, reload her area in Deeproot DepthsGrace to find D's brother and speak to him. Reload the area again to retrieve the Twinned Armor and the Inseparable Sword.