Is it balanced to create a homebrew sentient magic item that can attune to other magic items and grant the user their benefits?

Question

I'm imagining something like this:

A sentient, magical armor piece (I. E., the Mcguffin) that can store multiple weapons or shields to be summoned when needed. You are attuned to the Mcguffin, and any magical weapons stored in this manner are attuned to the Mcguffin itself. You only gain any bluffs and/or magical effects when the weapon is not being stored,

Maybe to prevent this from completely unbalancing a campaign, make it so that you need to gain the Mcguffin's loyalty to increase the number of items you can attune to it this way?

Example scenario: a mounted Paladin wielding a lance and shield gets thrown from his horse. When he gets up from prone, he uses an action to store the lance and shield, and summons his greatsword.

What I want to know is, is this kind of item completely game breaking, and if so, how can it be balanced?

Answer 1

This is unbalanced and the DMG warns against it

DND 5e was carefully balanced around a few assumptions. That player characters will only ever have access to 3 attuned items, one concentration spell and 1 bonus action per turn are some of them.

Chapter 9 of the DMG, p.263 contains the text:

Beware of adding anything to your game that allows a character to concentrate on more than one effect at a time, use more than one reaction or bonus action per round, or attune to more than three magic items at a time. Rules and game elements that override the rules for concentration, reactions, bonus actions, and magic item attunement can seriously unbalance or overcomplicate your game.

The difficulty of allowing an additional attunement slot is that is opens up a lot more options for game-breaking combinations of items. It may seem balanced now while they are just choosing between two different +1 weapons (though this isn't necessarily true). At later levels they may be able to attune to multiple ability increasing items as well as their armour and weapon which would make them extremely strong.

Answer 2

This is unbalancing

Assuming a character gains enough magic items requiring them to choose between several items, you can use more magic items than you could attune to at once in addition to being able to switch them using only an action instead of two short rests.

The following items could be used at the same time, all requiring attunement:

• The armor
  • Flame Tongue
  • Animated Shield
• Belt of Giant Strength
• Cloak of Protection

That's 4 items requiring attunement not counting the armor.

Furthermore usually if you only use a single weapon, you'd attune to a single weapon at the same time. Let's say you're in a combat with 2 enemies, one with immunity to fire damage and vulnerability to cold damage and the other with immunity to cold damage and vulnerability to fire damage.

In this fight you can be attuned to 2 other items and attack the enemies using the damage type they're vulnerable against by swapping between Frostbrand and Flame Tongue at the cost of a single action.

Loyalty and balancing

Maybe to prevent this from completely unbalancing a campaign, make it so that you need to gain the Mcguffin's loyalty to increase the number of items you can attune to it this way?

This is nothing balance-related. The fact that you need to do some roleplaying/quests to gain the benefits of the armor doesn't mean the item is not unbalancing once you're ensured the loyalty.

Assuming there are no other effects to the armor than being able to mess with the attunement rules, you should attune to another item as long as the armor's attunement limit is 1 making the armor effectively useless (you simply replace one attunement with another one and restrict the type of item you can attune to) and otherwise it's unbalanced, see above.