A heartseal can take many forms--typically they are small (2 to 4 inch diameter), round seals made from metal, wood, dragon scale, semi-precious stones, or other similar materials. The seal is embedded into the user's body, implanted into the skin of their chest, directly over their heart. A heartseal is clearly visible whenever the skin over the user's heart is visible. 

Heartseals are enchanted protection, a wondrous magical defense granted to those deemed loyal enough for the honor. Often heartseals are granted to a ruler's most powerful protectors, their closest friends--or a powerful 'ally' concerning whom they wish to have some insurance.

A fully functional heartseal confers the following benefits:

• Your chances of being hit with a critical hit are halved. Normally, this involves a second d20 roll. If you are hit with a natural roll of 20, roll a second d20. If the result is less than 11, the hit is still a hit, but the damage is not increased. If the attacker has a greater than normal chance of scoring a critical hit, that chance is halved as well. 
• You receive an extra death save upon reaching 0 hit points. 4 death saves are needed for you to die; you only need 2 successful saves to become stable. 
• Once per long rest, upon reaching 0 hit points (even if massive damage would kill you instantly), the heartseal stabilizes you. On your next turn, your hit points are set to 1. No save is required.

Requirement: In order to receive these benefits, at least one living being on your same plane of existence must know the command word for your heartseal. If anyone speaks the command word to intentionally invoke its power, your heartseal immediately stops your heart, resulting in your instantaneous death. You receive one death save in this event. Succeeding leaves you at 1 hit point. 

Heartseals, once implanted, may only be removed by a wish spell.