I like the idea that they have to find it and then take it to somebody: having braved a dungeon-full of monsters they then face the same creatures reanimated as they leave!
Consider locking it in a box which can only barely contain its power and sending it via a messenger on horseback to a victim. The messenger has a harrowing journey as he is beset by shadows and half-animated corpses. Upon delivery, the victim opens the box. While he is looking aghast at the beating heart, something unpleasant arrives, summoned by the heart, and kills him...Good for assassination purposes. Go to Comment
In my world, there is a large location after some kind of magical disaster, with a cemetery-gone-wild included. I know it horribly cliche, but... oh doesn't matter.
Now, if some hero ever manages to get into the center of that cemetery, what would he find? Some powerful undead creature? Or perhaps something more exotic? This item is one possible answer to this question. Although as an answer, it produces more questions than I know to answer.
Hmmm... assassin tool. Strange but possible. Makes an instant haunted mansion too. Go to Comment
It reminds me of the Heart of Lorkhan from the Elder Scrolls:
Lorkhan, the source of all instability, tricked the Gods into creating the world. As punishment, they tore out his heart and hid in the world. Later, the Dwemer found it, and used it to power their creations. Go to Comment
Plus it is magical, the rules of death are cheated by the magic armor regardless of the system you are in, have it do whatever fits your game. Give them real damage after -10 and let them die of unconscience pummeling (if people even do that) if that is your wish, or say the magic saves them, doesn't really matter in the scheme of things. Magic will do as magic does or as you desire. Go to Comment
If stolen, I assume that the beaten person would still be revived since the armor already did what it was suppose to, ie. convert it to subdual damage. So they would heal, albeit a little slower.
Give it to the enemy. Especially one that was difficult and has been causing them much problems. If they take the armor, fine, if not, then all the better. Either way, "Johnny's back." Great way to bring back your favorite villian. Convert whatever they are wearing to this. Ta da. Go to Comment
I like the conspiracy angle.
If anybody did find out about the armor and wanted to fake their own death....
Perhaps given as a gift to somebody who is then "assassinated" but really kidnapped.
Man in the Iron Mask type deal where somebody must be "murdered" to protect them from true murder. Go to Comment