Read from the World of Warcraft client build files (JournalEncounterItem, JournalEncounterCreature, Achievement) and the Blizzard Game Data quest API.
How Nightsfall Helmet Compares
Who It Suits
Plate is the armor type Death Knight, Paladin and Warrior wear. With Strength or Intellect, Nightsfall Helmet suits Arms, Fury and Protection Warrior; Holy, Protection and Retribution Paladin; Blood, Frost and Unholy Death Knight.
Transmog Sets
Nightsfall Helmet is part of the transmog set Leyline Defender's Sunplate Armor (Warrior, Paladin and Death Knight only). The other 16 pieces:
Blizzard numbers items as they are created, so the named items next to ID 139058 (139048 to 139063) were usually added in the same batch as Nightsfall Helmet: the same quest line, zone, vendor or reward set.