From one end protrudes a fine needle, which easily enters the flesh of any creature applying even the slightest amount of pressure. If an attuned individual does this, magic draws a single drop of blood from them, which is suspended within the vial. If only one creature does this, then the item has no effect, for it needs at least two creatures to add their blood, in order for it to function.
When more than one creature adds a drop of their blood, their life force (in the form of hit points) become combined, meaning that instead of having their own separate hit points, they add their totals together, and any damage they suffer comes from that shared pool of hit points. This means that should the pool be depleted, all connected creatures are rendered unconscious, and begin to die.
Healing works in the same way, restoring lost hit points to the pool.
Whenever any hit points are lost from the pool, all connected individuals feel it as pain. They may, as a bonus action, attempt a D.C. 10 Wisdom ability check to see if they can determine who has been wounded.
Temporary hit points still only benefit the one gaining them.
There are some limitations. If any creature is separated from the vial by more than 100 miles, the link is broken. Similarly, if a creature is on a different plane to the vial, the link is lost. No more than three creatures can be linked to the vial, and if any are exposed to any effects that can cause instant death, all linked individuals are similarly exposed to it.
|"Billy Bob? That vial o' yer blood ye giv me', be glowin' all spooky like agin!"|