Ghost Gem

Wondrous Item, uncommon
While looking like an oversized glass bead when empty, this gem is composed of crystalized magical force. You can use an action to make a melee attack against an undead creature with the incorporeal movement ability or the amorphous ability of a shadow (or similar undead). If you succeed, the creature must succeed on a DC 15 Charisma saving throw or be trapped within the gem. The trapped creature can be seen, albeit in a distorted way, within the gem. A trapped shadow makes the gem look as if it is filled with black ink while a gem holding a ghost might seem to contain a faintly luminous grey fog. The creature’s features can sometimes be seen, perhaps a tiny, twisted ghostly face pressed against the gem or ghostly talons scraping the gem. A trapped creature is aware of what is occurring around it, within the limits of its senses. It can, however, take no actions or reactions while contained. If it can make sounds, these can be heard by pressing the gem against your ear; this can allow communication with a trapped creature.
A trapped creature can be released by speaking a command word while holding the gem or by breaking the gem. A ghost gem can be used as a sling stone, doing damage if it hits a target and taking the same damage. A ghost gem is AC 18 and takes 3 points of damage before shattering. A broken gem cannot be reused. You have no special control over a released creature; it will act in accord with its nature.
Ghost gems are sometimes used in traps when having free roaming undead would be undesirable. Some assassins use these to make it appear that their target was the unfortunate victim of a random attack by undead.