Sets

The full set gives not stats but a combat property: a shield at the start of a fight, a strike bonus or lifesteal. That is why four parts of a set usually beat four random items of the same rarity.

A land may have a set of its own that drops nowhere else: the Attire of the Sleepers falls only from the creatures of the Swamp of the Sleepers — not in dungeons, not from chests.