Back in the 3.5 DMG II, there were magic items that benefited the whole party. I don't remember the details of their design, and I'm deliberately not going back to look them up now, but I always liked the idea - company banners, totems, that kind of thing. I was reminded of this in a D&D session I played yesterday, part of the Reborn campaign. We had gotten a jade statuette, the effect of which was to cleanse the air of poison around us in a 20-foot radius, as long as someone maintained Concentration on it. This was a mechanic to add some environmental threat to a dungeon, but it sent me off in another direction of ideas.
Specifically, it made me think about items that allow and encourage more than one party member to attune to them at a time. In my Tribality article series on the fey, I experimented with this idea some time back:
Phial of Elder Starlight
Potion/Wondrous item, very rare (requires attunement)
While you have this phial attuned and stoppered, it grants a +2 bonus to all saving throws and sheds pale blue light. This provides bright light to a radius of 30 feet and dim light 30 feet beyond that. Up to ten creatures may simultaneously attune to the phial, but it does not grant a saving throw bonus if more than one creature is attuned to it. When you open the stopper as a reaction, the elder starlight pours forth and fills a 60-foot radius centered on the phial. When a creature that is not attuned to the phial starts its turn in the area of elder starlight, it must succeed a DC 19 Constitution saving throw or suffer 3d10 radiant damage. On a successful saving throw, the creature takes half damage. This area fades after one minute.
But, well, that doesn't exactly explore the full reach of this idea, does it? I'm interested in capturing some of the party-leader feel that made 4e leaders so much fun to play, but using magic items and attunement as a justification. Here are a few more ideas I had; design notes to follow.
Weapon (glaive), legendary (requires attunement)
Up to eight characters may attune this gleaming silver glaive at any one time. The glaive holds up to 8 charges, and any attuned creature may use a bonus action to expend a charge and teleport to an empty space adjacent to any other attuned creature. The first time they deal damage before the end of that turn, they deal an additional 1d6 radiant damage.
The glaive sheds bright light in a 40-foot radius and dim light for an additional 40 feet.
An attuned creature wielding the glaive gains a +3 bonus to attack rolls, adds 1d6+3 radiant damage to damage rolls made with this magic weapon, and gains a fly speed of 50 feet from silver-white feathered wings that sprout from the shoulderblades. When this glaive deals damage that reduces any celestial, fiend, or undead to 0 hit points, it regains 1 charge.
Crimson Bull Skull
Wondrous item, very rare (requires attunement)
Up to eight characters may attune this horned skull at any one time. Attuned creatures gain a +3 bonus on all Strength and Constitution saving throws, and gain one virtual hit die, a d12, that does not increase their maximum hit points, but that they may spend to regain hit points as part of a short rest.
An attuned creature carrying or wearing this skull, which fits very large orcs, ogres, and trolls as a helmet, regains 3d10 hit points when any other attuned creature fails a death saving throw or dies. The creature carrying or wearing the skull may impose disadvantage on the death saving throws of other attuned creatures at will. As a reaction, the creature may spend a hit die and add the result to any attuned ally's death saving throw result.
Fulminata Legion Banner
Wondrous item, very rare (requires attunement)
Up to eighty characters may attune to this elaborately decorated legionary banner at any one time. A creature who becomes charmed, or who deliberately deals damage to another creature attuned to this banner, loses attunement. All creatures attuned to this banner receive an aid spell, as if cast with a 2nd-level spell slot, that is constantly active. If this effect is dispelled, it is renewed on the following dawn.
An attuned creature carrying this banner (which is large enough to require both hands) may expend a charge to cast call lightning as a 3rd-level spell without expending a spell slot. Creatures attuned to the banner automatically pass saving throws against this spell, and take no damage. The banner holds up to three charges, and regains all charges when exposed to a natural thunderstorm for at least one hour.
Paladin King's Crown
Wondrous item, very rare (requires attunement)
A number of characters equal to the the wearer's Charisma modifier may attune to this crown in addition to the wearer. All creatures lose attunement to the Paladin King's Crown when it is removed for more than three rounds. Attuning to the crown requires only a bonus action, in which the wearer makes certain sacred gestures, or non-wearers genuflect in the direction of the wearer.
All attuned characters gain advantage on saving throws against the charmed condition. A creature that already has this benefit from some other feature or effect instead adds 1d6 to the saving throw result. Any attuned creature with the Lay on Hands class feature may use it to heal other attuned creatures at a range of 30 feet, rather than touch.
The crown has 7 charges and regains 1d6+1 charges at dawn. As a bonus action, the wearer may spend a charge to end the frightened condition on one creature within 30 feet that is attuned to the crown, and the creature regains 2d6 hit points.
Wondrous item, legendary (requires attunement)
Any number of sentient creatures may attune to this statuette, which is made of no earthly stone nor metal, at the same time. All attuned creatures with Intelligence scores of 3 or higher may communicate telepathically with one another, regardless of distance, as long as they are on the same plane of existence. They do not need to share a common language. Further, they gain resistance to psychic damage, unless the effect's source is attuned to the tentacled statuette.
An attuned creature carrying or touching the tentacled statuette may treat any attuned creature's location as very familiar, for purposes of scrying, teleportation, or other spell effects. As a bonus action, a creature carrying or touching the statuette may inflict 2d6 psychic damage on any attuned creature to heal itself for an equal amount of damage. It may not inflict this damage to the same creature again until the target creature completes a long rest.
A group of up to three attuned creatures may cause a helpless or charmed creature to become attuned to this item in one minute. Ending attunement to this item requires physical contact with the statuette and a remove curse spell. This item almost certainly has other effects. You will sleep better at night if you just say to yourself, "I'm sure it's fine."
If you have played Dust to Dust for any great amount of time, you probably know what the archangel's glaive is all about, though this is more a thematic than direct adaptation. Anyway, I loved that paladins in 4e got angel-themed powers to teleport people around the battlefield, so I wanted to reference that here. This should feel like one of the greatest magic weapons in the setting, on par with the holy avenger sword; if it falls short of that, I might revisit it. It recharges from killing celestials because I figure the Fallen are still Type: Celestial.
The crimson bull skull is all about the selfishness of orc, ogre, and troll chieftains in hordes. It makes their followers better. It also makes them into little first-aid kits. Join the Horde! Serve your chieftain! Twice! For real horde-like mechanics, I would need to greatly increase the number of creatures that can attune to it at once - I might offer a second, lesser effect for creatures after the eighth that attune to it. This one is useful to PCs, but really intended for NPCs.
The Fulminata legion banner is kind of the quintessential concept of the whole post - everyone is attuned, everyone is protected, the bannerman gets an extra option. Tossing out aid on up to 80 people should have enormous effects in a mass-combat system, as should the call lightning spell itself.
The Paladin King's crown is another Dust to Dust reference, but this time is about the highest ideals of enlightened kingship - protecting the will and easing fear and wounds. Lay on Hands still takes an action, so getting to use it at a range can't really be all that broken. I liked the idea that the monarch didn't have to be a paladin to get that effect - any of the attuned could protect the monarch from afar thanks to this item.
The tentacled statuette probably doesn't need a lot of explanation, but I was trying to write a magic item that would be the center of an extended storyline of Lovecraftian horror. I spelled out all of the effects that I knew I wanted, and left anything else as an exercise to the DM.
I expect that in a lot of cases, it's a situational choice whether you want to attune to the team magic item or not. There are plenty of magic items that are individually better than the effect non-bearers receive from these items, but probably some characters reach mid-to-high levels without three attuned items they care about using.