The Sentinel Set (an excerpt from Ars Artifexam)
The Sentinel Set (various, very rare, require attunement)
This quartet of items was crafted carefully to work together, and each individual piece becomes more powerful, the more pieces of the set one has. Even though the set has four pieces, and all require attunement, the fourth piece may be attuned as long as the first three have been.
The First Piece is the Sentinel Scutum. Like its more common cousin, this is a shield, in this case usually a large rectangular convex shield. Like its cousin, it grants advantage on Wisdom (Perception) checks and initiative rolls. If two pieces of the Sentinel Set are attuned, it gains a +1 AC and when a creature you can see attacks a target other than you that is within 5 feet of you, you can use your reaction to impose disadvantage on the attack roll. If three pieces are attuned, you gain the Shield Master feat. With all four attuned, it gains a total of +3 to AC.
The Second Piece is the Sentinel Spear. It grants you a +1 to hit and damage, has the finesse quality, and can be extended to as long as 10', gaining reach. When paired with the Scutum, it can be wielded one handed for 1d8 base damage, and grants advantage on attacks of opportunity. With three of the set attuned, the wielder gains the Sentinel feat. With four items attuned, the wielder gains the Polearm Master feat.
The Third Piece is the Sentinel Helm. Usually this resembles a Roman galea or a Corinthian helmet. Initially, it grants you Darkvision 60' and gives you advantage on Wisdom (Insight) checks. With two items attuned, you gain Devil's Sight, and can see through even magical darkness, as well as gaining advantage on Intelligence (Investigation) checks. With three items attuned you become immune to the friends spell, and gain advantage on all saves vs charm. With all four items you gain truesight 60'.
The Fourth Piece is the Sentinel Armor. Usually lorica segmentata (splint) or a breastplate, but can be any type of armor. By itself, it grants a +1 to AC, and you gain the Tireless Watchman trait: You don't need to sleep, and magic can't put you to sleep. When you take a long rest, you must spend at least six hours in an inactive, motionless state, rather than sleeping. It doesn’t render you unconscious, and you can see and hear as normal. With two pieces of the set, you gain the Armor Mastery feat appropriate to the type of armor, usually Medium. Three pieces of the set grants you resistance to nonmagical bludgeoning, piercing and slashing damage. Four pieces of the set grant you immunity to the frightened and charmed conditions.