Overpowered?

i dont think so. you can still be killed by enough damage to take you to negativeyour max health, by things attacking after you've fallen but before you've stabilized, disintegrate, etc. not to mention that it requires attunement and does nothing for u when u dont need to be stabilized or if ur not healing from hit dice. not saying those aren't extremely useful, but many people would probably replace this item with something that aides their abilities at some if given the choice.

Personally, I find that imposing a point of Exhaustion each time this stabilization occurs helps balance this item. If they go down and up 6 times they die anyway... Food for thought. This item is incredibly useful for parties from Levels 1-7, afterwards it has diminishing returns. 

Also notice that is heals from hit die. This doesn't include potions.

Awesome combined with Dwarven Fortitude racial feat.

@obadiah80

Thinking the same thing myself. Throw durable in for good measure.

When exactly does a character: " roll a Hit Die to regain hit points"?

Short rest

@Milanthron Or with the feat "Dwarven Fortitude" you can do after taking the dodge action.

Can anyone tell me the cost of this item? Because I know it doesn't cost 10,000 gp

I wouldn't call it overpowered either, but I personally would imagine the rarity being quite higher. 

375 gp

Discerning merchant's guide prices it there.

Magic items don't usually have a cost... however if you go by ZGE it says it should cost between 100-500g pieces however this is up to your DM and your persuasion skill. Also you might not be able to buy this at all...

Oh, so this is Excalibur’s sheath.

Quote from Dr_Frederick >>

Oh, so this is Excalibur’s sheath.

 I want to upvote this response lol.

Stable doesn't mean awake after being knocked out. 

What happens when you're dying underwater?
From the Suffocating rules "At the start of its next turn, it drops to 0 hit points and is dying, and it can't regain hit points or be stabilized until it can breathe again."?

Does this double just the die result? Or does is double the HP regained from spending the hit die? Hit die is the 1d# + CON, so its 2(1d#)+ CON or is it 2(1d#+CON)?

Since you cannot be stabilized while suffocating The Periapt of Wound Closure would not be able to take effect and you would start making death saves.

Holy hell... This thing IS overpowered. I thought it only stabilized you once per long rest, but it just KEEPS DOING IT. And it only costs upwards of 500 gold?