1. Let’s say there are magic items that grant one daily use of any one of the maneuvers listed for the Battlemaster on page 74 of the Player’s Handbook.
2. The item would magically carry one superiority die (a d8) that you can use when you activate the item as part of taking your attack action, or whenever spending the die is called for. You get the die back the next day, at dawn.
2. All of these particular items require attunement, and one of the quirks of these items is that if you are attuned to any other item in the set, then attuning to a second item does not use up one of your attunement slots. In effect, you are attuned to the set.
3. Oops, “quirk” isn’t a good word to use. What you should write is Minor Property. Keep it simple by giving it a simple name: [B]Set Attunement[/B].
4. Now if you’re already a Battlemaster when you acquire one of these bad boys, the superiority die granted by the magic item is the same as your regular superiority dice. So if you’re a badass mothertrucker 15th level Fighter (Battlemaster), that’s a d10 you’re working with for all of your set items.
EDIT (just had an idea): 4.5. If you acquire and equip at least four items in a set, then all of the superiority dice that come from your set items recharge at the same rate as your regular superiority dice, so after a short or long rest.
5. If you manage to acquire and equip the entire set, then all of your superiority dice–whether from your items or from your Archetype–are jacked up one notch. Therefore, if you equip the whole set at, say, 17th level, your superiority dice are d12s and not d10s. When you level up to 18th, they become d20s!
BOOM goes the hammer/sword/whatever it is you’re wielding!
6. Looking for all the pieces would be like hunting for the Paladin’s Holy Sword, but WoW style, and so presumably a lot more fun, and without all of the stupid moral conundrums that make Holy Avenger hunting such a drag.
7. You’ll need to dig deep and produce good Realmslore if you make this idea part of your Cormyr sourcebook. Maybe Baron Thomdor had a set. Or Duke Bhereu. Or one of the dead noble lords of Esparin (see this post) had some pieces, and now it’s inside a Metal Keeper (see this post).
8. There are what, 9 or 10 places where magic items can be worn on the body before things start to overlap? That is, maximum number of items worn without invoking your DM’s soul-withering stink eye. And there are 16 maneuvers for the Battlemaster.
9. So maybe you can build two sets of 8 magic items, and each set is built around a theme that all of the maneuvers in the items fit with.
10. No…here’s what you do: create 2 new maneuvers for the Battlemaster (so 18 maneuvers available to choose from) and make the actual maneuvers Cormyr-themed in some way–like a Purple Dragon-inspired maneuver that lets you aid another PC’s Grapple or Shove: They are considered one size category larger for purposes of adjudicating the results per PHB 195–and then create 3 sets of 6 items each.