Would this affect armor?

I doubt it.

It would not help if your speed was reduce because of armor. Nor would it help if your speed was reduced because of weight (encumbrance)

Does anyone know the price?

Quote from Fish_Gnoll >>

Does anyone know the price?

 501-1000 i think

Does this prevent slow spell from taking effect when worn.

• You alter time around up to six creatures of your choice in a 40-foot cube within range. Each target must succeed on a Wisdom saving throw or be affected by this spell for the duration. An affected target’s speed is halved, it takes a −2 penalty to AC and Dexterity saving throws, and it can’t use reactions. On its turn, it can use either an action or a bonus action, not both. Regardless of the creature’s abilities or magic items, it can’t make more than one melee or ranged attack during its turn. If the creature attempts to cast a spell with a casting time of 1 action, roll a d20. On an 11 or higher, the spell doesn’t take effect until the creature’s next turn, and the creature must use its action on that turn to complete the spell. If it can’t, the spell is wasted. A creature affected by this spell makes another Wisdom saving throw at the end of its turn. On a successful save, the effect ends for it.

• It would definitely nullify the reduction in movement speed. If I were DM I would grant full immunity to the whole spell but it's ultimately up to whoever's running your game.

Does sentinel feat effect this

"In addition, magic can neither reduce your speed nor cause you to be paralyzed or restrained."

The key word here is 'magic.'

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"You have mastered techniques to take advantage of every drop in any enemy's guard, gaining the following benefits.

• When you hit a creature with an opportunity attack, the creature's speed becomes 0 for the rest of the turn."

Since the Sentinel feat is reliant upon one's skill/technique in combat and is not magical in nature, the aforementioned feature of Sentinel would still affect you even if you wore the Ring of Free Action.

Just to clarify, does this affect plant growth?

Quote from Keltsune>>

"In addition, magic can neither reduce your speed nor cause you to be paralyzed or restrained."

"You have mastered techniques to take advantage of every drop in any enemy's guard, gaining the following benefits.

• When you hit a creature with an opportunity attack, the creature's speed becomes 0 for the rest of the turn."

Since the Sentinel feat is reliant upon one's skill/technique in combat and is not magical in nature, the aforementioned feature of Sentinel would still affect you even if you wore the Ring of Free Action

 Seeing this I still cannot beleive nobody has realized that ordinary Caltrops and Ball Bearings will still work on a character with this ring.

Since the ball bearings dont create an area of difficult terrain, but instead force you to make a Dex save if you decide to move your full speed they are unaffected by this resistance. Similarly Caltrops have another moving full speed means you make a save or take damage and stop moving.

This item is very similar to the Freedom of Movement spell, except the spell has the additional capability as follows:

The target can also spend 5 feet of movement to automatically escape from nonmagical restraints, such as manacles or a creature that has it grappled. Finally, being underwater imposes no penalties on the target's movement or attacks.

Would be really nice if the item had the same capability

The name gives me an idea for this item's counterpart: the Ring of Literal Free Action which gives you an action surge once per dawn.

I feel like that would be at least a rare probably a very rare level item as though fighters do get it at second level it’s still a very powerful ability to give to any other class. Also never leta fighter ever even touch that ring the amount of attacks will drive you insane.

So after reading this comment (POST #13), I can see how these 2 things are the same, but different. They should have made them with different wording.

Just to clarify, as one of my players has a ring, anther has the spell -

Ring: No effect given for spider webbing that can cause the restrained condition or anything similar. It only affects difficult terrain or magical impediment conditions.

Sad that you cant say no to the sentinel feat while wearing it. Still looking though.

If you do know a way (other than getting the feat itself) please tell me.

they should i named this "the ring of free movement" if they're going to change what the ring does they should also change the name

So, a Weapon Mastery effect that slows the target (like on a longbow) would work, but a Slow spell would not?  I don't know, I think that I'd say the ring and a Freedom of Movement spell would allow a target to ignore ALL reductions in speed, not just magical and environmental causes.

Quote from MacBain413 >>

Just to clarify, does this affect plant growth?

 no. Plant Growth create non magical effect and does not make "difficult terrain".