After Shardbearer Radahn (main story boss) is defeated, head back to the Redmane Castle. You will have gone through there to reach the Radahn boss. Talk to the old man sitting on a chair in the Church a few times until all dialogues are exhausted. Then fast travel away and come back here, a yellow mist gate should now spawn. Go through it and a boss fight will trigger in the courtyard against a double boss “Misbegotten Warrior” & “Crucible Knight”. Defeat them and they will drop this armament.