Mark Of Nobility

[JNAProductions]

Mark Of Nobility
Rare Wondrous Item, Can Be Attuned

This large mark appears similar to a tattoo, and is usually given to the noblefolk shortly after birth. Simply by having this, the person is marked as nobility, with different markings indicating different noble lines. When the mark is used or during times of significant stress, the mark glows. Additionally, the bearer of the mark has a +1 to all Strength, Dexterity, and Constitution checks.

If attuned, the mark gains significantly more power. It has three focuses, each granting different effects. The focus may be switched over the course of any rest. You may also switch the focus as a bonus action once, and regain the ability to do so upon completing a short or long rest. The focus may be expended, granting a significantly increased effect, but un-attuning the Mark after the effects are completed.

Strength Focus
When focused on Strength, you gain the following effects:
-Your Strength score increases by 4, to a max of 20.
-Your carrying capacity is doubled.
-You add your proficiency bonus to melee weapon damage rolls.
Expending the Strength Focus grants the effects of Enhance Ability (Bull's Strength) and Enlarge in addition to the existing effects for one minute, without concentration being required.

Dexterity Focus
When focused on Dexterity, you gain the following effects:
-Your Dexterity score increases by 4, to a max of 20.
-You have resistance to falling damage.
-Your move speed increases by 5'.
Expending the Dexterity Focus grants you the effects of Enhance Ability (Cat's Grace) and Haste in addition to the existing effects for one minute, without concentration being required.

Constitution Focus
When focused on Constitution, you gain the following effects:
-Your Constitution score increases by 4, to a max of 20.
-You reduce any instance of damage taken by your proficiency bonus.
Expending the Constitution focus grants you the effects of Enhance Ability's (Bear's Endurance) and Stoneskin in addition to the existing effects for one minute, without concentration being required.

[Catullus64]

I'd say it looks like a very fun item, but far too strong for a Rare. If you want to keep it as is, I would make it Legendary, Very Rare at the least. This item grants:

• +4 (!) to a physical ability score, though no raised maximum, and you have a lot of flexibility in changing it around (short and long rest, and an instant change between)!
• +1 to a fairly wide array of ability checks, including initiative.
• Passive features that people sometimes take entire feats & fighting styles to get worse versions of (Flat DR surpassing Heavy Armor master, flat damage bonus far surpassing Dueling Fighting Style).
• Very potent abilities on breaking attunement, during the duration of which you still benefit from attunement.

By contrast, the best analogue, the Manuals of Bodily Health, Quickness in Action, and Gainful Exercise (all Very Rare) grant:

• +2 to an ability score, permanent, with a maximum increase, but you can't change it around.
• That's it.

If you want it to remain Rare, I would recommend making it into three separate items, one for each ability score, toning the ability score increase down to 2, and significantly pruning back the passive buffs. You should also specify what kind of action, if any, is required to break the attunement. At the very least you should restrict it to being on your turn.