I (the GM) currently have a wizard in my campaign that was able to nab an amulet that controls a shield guardian. If an enemy were to make an attempt to break the amulet's chain so that it was no longer being worn (thereby causing the wizard to lose control of the shield guardian), what would the mechanics be for that? Would they make an attack roll against the wizard's normal AC, or would the AC be higher since it's a smaller, more specific target?
Objects have HP and AC
According to the rules as they written. Objects have an AC and HP like characters do. So you can make attacks against items.
Object Armor Class
- Substance AC0
- Cloth, paper, rope 11
- Crystal, glass, ice 13
- Wood, bone 15
- Stone 17
- Iron, steel 19
- Mithral 21
- Adamantine 23
Object Hit Points
- Size Fragile Resilient
- Tiny (bottle, lock) 2 (1d4) 5 (2d4)
- Small (chest, lute) 3 (1d6) 10 (3d6)
- Medium (barrel, chandelier) 4 (1d8) 18 (4d8)
- Large (cart, 10-ft.-by-10-ft. window) 5 (1d10) 27 (5d10)
There is no reason you couldn't use the standard grappling rules against a worn object (Grapple specifically mentions creature, but the mechanics could just as easily apply to objects as they already have AC to test against); possibly with disadvantage for movement or AC bonus for cover.
I, personally, would likely also make breaking the chain a subsequent athletics check or attack on the next turn, so that the player has a chance to attempt to stop the effect between you getting ahold of the item, and breaking the chain.
Or, Follow the Example of the Ioun Stone
The Ioun Stones have very specific rules about being taken or being broken. You could just adapt them to your necklace.
...must use an action to grasp or net the stone to separate it from you, either by making a successful attack roll against AC 24 or a successful DC 24 Dexterity (Acrobatics) check. You can use an action to seize and stow the stone, ending its effect.
A stone has AC 24, 10 hit points, and resistance to all damage. It is considered to be an object that is being worn while it orbits your head.
Use an Improvised Action
You do this by using the Improvising An Action rules, which exist to cover everything that isn't covered by a specific action. You can use it to resolve an "I snatch the amulet" or "I break the chain" attempted action. You'll find them in the inset box at the top-right of page 193 of the Player's Handbook. These rules refer to the DM telling the player what to roll — as DM, you'll be referencing the general guidance in the DMG in the Using Ability Scores section that starts on page 237.
Basically, the Improvising An Action's rules point that “The only limits to the actions you can attempt are your imagination”, and D&D 5e's emphasis on simplicity and speed, means that you pick an appropriate ability score, set an opposed target (either a DC or an ability score roll made by the target), and then roll 'em to see if it worked. Advantage and disadvantage as usual are applicable, and for such a specific and small target, probably relevant.
PCs can do what NPCs can do
As a bonus, using the improvised action rules will show them off to the players. Perhaps they thought their only options were the actions listed in the PHB! Seeing the amulet snatched may inspire them to start doing more varied things during combat — which will make fights in your game more dynamic, creative, and interesting.
This also gives you a way to judge how hard it should be for your NPC: how hard would you want this maneuver to be if the PCs tried it? So make it neither too easy nor too hard, and it will improve your game in the long run (even if the NPC's snatch attempt fails!), because it will enrich your players' imaginations.
(To that end I would personally rule this an opposed Dex vs. Dex roll, but with disadvantage to the snatcher due to the small size of the target. That means it has a small chance of success against all but very clumsy opponents, which seems about right. It also fits the "value" of the attempt as very high: a risky, low chance of success, but with a high payoff is a nice balance.)
D&D 5e (and previous versions, for that matter) has no mechanism for such specific targeting. It has always been assumed that a person's carried belongings are (perhaps even magically) a part of him, and if the character survives, his stuff survives with him, almost always.
So, that being said, snatching an amulet from an active person is not something there is provision for. To target the amulet without breaking the spirit of the game will require that the mage be rendered in a condition of unconscious, paralyzed, or at the very least, restrained, such that the amulet can be removed from his person.
Treat the amulet as different entity that can be attacked.
This is how I will determine the DC (instead of AC):
- The amulet is unlikely to be kept within armor, so it does not benefit from armor AC.
- The wearer can try to evade the attack, so it will benefit from DEX modifier when wearing armor. So if the target wear a medium armor, it will have +2 maximum.
- Because he is targeting a specific part of the "body", increase the DC by 5.
For example with +2 DEX, that makes the DC = 10 + 2 + 5 = 17
This is how I decide if his attack successfully hit the amulet:
Do attack roll with disadvantage. If the attack is successful against the DC, then the amulet is destroyed. If not, then use the disadvantaged attack roll to attack the target character as usual with their AC.
You missed the amulet by inches, but your sword barely managed to land on his shoulder instead.
You do the attack with disadvantage because you aim for specific part instead of freely looking for openings.
In my campaigns, when the PC's target an item, they have to make a dexterity check to see if they hit it. If they hit, they make a normal attack against the item, including the item's AC. Assuming the item is being worn, such as an amulet, it should be assumed that the person wearing said item should be hit by the attack as well. Therefore, the PC makes a standard attack against the wearer, including the wearer's AC. Items may be large enough to grant the wearer additional protection, such as granting additional AC to the wearer, but such things are totally up to the DM.
The principle of attacking an item worn or otherwise attached to a player can also be applied to ropes or chains that restrain or bind a character. Players should take disadvantage to hit the person that is restrained, since, chances are, they're trying to free them without harming them. The disadvantage represents the possibility of a player slipping up, and accidentally harming the character they are trying to free.
In one of my campaigns, one of my players put on a suit of armor that was, unbeknownst to him, cursed. The armor trapped him inside, and made him immobile. Another player attempted to attack the armor in creative ways as not to harm the player wearing the armor. They eventually destroyed said armor with minimal harm done to the player that was wearing it.