How can I make a Animated Armor Trap?

Question

In the Dungeon Master's Basic Rules, one of the monsters that was featured is Animated Armor. I was disappointed that they did not address the possibility of intelligent animated armor attempting to lure a PC into trying to wear it and then taking control of said PC. So when I brought home my shiny new Monster Manual on Saturday, I was even more disappointed that this was still not addressed explicitly, forcing me to come up with my own mechanics.

My desire is to use Animated Armor as a trap; My players are smart enough to cast detect magic on fancy new treasure and have the resident spellcaster carefully inspect things that appear too good to be true, but they are also greedy and "completionists." Despite their caution, they can never resist a shiny piece of treasure. I want to exploit this behavior by having a suit of Animated Armor lie in wait and allow one of them (hopefully the party tank) to put it on.

1. Should the Animated Armor's control over the PC be implemented via the grapple mechanic? Is that technically feasible? If so, I feel like either the PC should get disadvantage or the Armor should get advantage. Is there a significant difference statistically in the outcome depending on which one of those I choose?
2. If the grapple mechanic is used to implement this tactic, would it be unfair to grant the Armor the "Damage Transfer" property of the Rug of Smothering? I feel like attacks directed at the armor should also harm the wearer, but effectively granting the armor complete damage resistance (allowing it to only take half damage from virtually all attacks) seems pretty brutal.
3. While the Armor is controlling a PC, should the "combined creature" (combination of PC and Armor) attack using the PC's attributes and attacks or the Armor's attributes and attacks? For example, take a fighter who wields a Pike who has been taken over by a suit of Animated Armor. Can the Armor make attacks with the Pike using the fighter's Strength, or is the Armor forever fated to only be able to use it's default Slam attack with its own Strength score?
4. Similar to #3, should the combined creature use the PC's AC or the Armor's AC? For example, assume a fighter wielding a shield and a Ring of Protection +1 straps on the Animated Armor and loses his grapple check, his total AC would be 21 (base 18 from Full Plate Mail, +2 from the shield, +1 from the Ring of Protection). Should attacks against the PC+Armor be targeted against an AC of 18 (for just the Armor) or 21 (for the PC)?

Answer 1

While at first it doesn't seem an efficient trap given the fact that animated armor will need to spend part of its time establishing control. It does have a nasty aspect that the PC will be inside forcing the rest of the party to come up with a innovated way of stopping the armor without hurting (much) or killing the character inside.

Should the Animated Armor's control over the PC be implemented via the grapple mechanic? Is that technically feasible? If so, I feel like either the PC should get disadvantage or the Armor should get advantage. Is there a significant difference statistically in the outcome depending on which one of those I choose?

Basically yes, at its core should involve a contest between the two opponent's strength so the grapple use of the strength based skill Athletics is a good choice. However I would rule that can't use Acrobatics once the PC is inside the armor. Only Athletic checks are allowed.

The same with the escape attempt, once inside the only way to regain control is with a Athletic (Str) check.

If the grapple mechanic is used to implement this tactic, would it be unfair to grant the Armor the "Damage Transfer" property of the Rug of Smothering? I feel like attacks directed at the armor should also harm the wearer, but effectively granting the armor complete damage resistance (allowing it to only take half damage from virtually all attacks) seems pretty brutal.

The Damage Transfer ability of the Rug of Smothering means that it takes longer to kill the creature. The target inside the Rug will take the same amount of damage needed to kill the Rug itself.

Now that I think about it the Damage Transfer ability was probably granted to the Rug in order to prolong the danger from the Rug's Smother attack. In light of this my recommendation is that for the Animated Armor trap, damage be equally allocated between armor and the character inside. However remember the character inside doesn't have the poison or psychic immunity the armor has.

While the Armor is controlling a PC, should the "combined creature" (combination of PC and Armor) attack using the PC's attributes and attacks or the Armor's attributes and attacks? For example, take a fighter who wields a Pike who has been taken over by a suit of Animated Armor. Can the Armor make attacks with the Pike using the fighter's Strength, or is the Armor forever fated to only be able to use it's default Slam attack with its own Strength score?

No that doesn't make logical sense. The way you describe it the Animated Armor is acting like an sci-fi exoskeleton with a will of its own.

Similar to #3, should the combined creature use the PC's AC or the Armor's AC? For example, assume a fighter wielding a shield and a Ring of Protection +1 straps on the Animated Armor and loses his grapple check, his total AC would be 21 (base 18 from Full Plate Mail, +2 from the shield, +1 from the Ring of Protection). Should attacks against the PC+Armor be targeted against an AC of 18 (for just the Armor) or 21 (for the PC)?

Basically yes except that the character inside would not get a dex bonus to their AC or use of their shield.. If they still have a higher AC, I would rule that the only the Armor suffers damage if the attack is equal to or greater than the Animated Armor AC but less than the PC dex-less AC. If the PC had a shield it would be used by the Animated Armor making it a AC 20 but not the PC.

Final Thoughts

I would rule it like this

1. The PC puts on the Armor
2. The armor comes to life initiates a grapple check.
3. If the PC succeeds, he yells help and we go back to step 2. The PC will get his normal turn.
4. If the PC fails, he goes oh crap and still yells help, and now has the restrained condition similar to the result of the Rug of Smothering's smother attack.
5. The armor uses it interaction to pick up the PC's weapon. If the PC has a shield then it will spend its action to pick up both weapon and shield. The armor then moves and does it action (if has one)
6. PC can attempt an escape with Athletics only.
7. The armor can takes it turns as long as the PC doesn't succeed on an Athletic check. The PC does then the Armor will return to fighting for control.
8. If the PC makes a number of successful Athletics, your call, then he has managed to rip the armor. If the party aids him then he makes the check with advantage. I recommend at least two in a roll, no more than four in a row.

Answer 2

If the armor could never use the stats of the person who is wearing it, why would the armor care about being worn at all? It gains no benefit from being worn, only downsides (it now has to compete with somebody else for movement).

Animated armor has stats because it is assumed to be animated (move on its own). Your version of animated armor sounds more like you're trying to create some sort of cursed armor which will try to take control of the person wearing it, in which case it doesn't truly need stats of its own, it's controlling somebody else so it enjoys the benefit of having the statistics of a scary hero.

As for the mechanics to implementing this control and how to damage the creature, it is most likely a lot more effective if you implement it as a mind-altering effect than a grapple. Not only is the kind of character who tends to wear armor going to be far more likely to fail at resisting a mind-altering effect than a grapple, but it also makes a lot more sense. A single grapple result would indicate the armor "overpowered" the warrior and keeps him grappled, but that doesn't suddenly allow it to control all its movement. Armor and fighter would be in constant struggle against eachother, making it very hard to fight other targets. They are -both- under the grappled effect, the armor has no real action except to continue to hold the grapple.

Answer 3

Bit of a Lengthy answer but bear with me.

I agree with @Theik in that it sounds more like you're wanting to create cursed armour, but since there aren't rules to cover cursed items yet we'll run with your idea.

With the lack of a Dominated condition in 5e I believe either Charmed or Restrained could work as well so to put forth another way it could be done let me direct your attention to the spells on;

Page 235 of the PHB

Dominate Person:

You attempt to beguile a humanoid that you can see within range. It must succeed on a Wisdom saving throw or be charmed by you for the duration. You can use your action to take total and precise control of the target. Until the end of your next turn, the creature takes only the actions you choose, and doesn't do anything that you don’t allow it to do. During this time you can also cause the creature to use a reaction, but this requires you to use your own reaction as well.

and page 251

Hold Person:

Choose a humanoid that you can see within range. The target must succeed on a Wisdom saving throw or be paralyzed for the duration. At the end of each of it's turns, the target can make another Wisdom saving throw. On a success, the spell ends on the target.

Once the PC completely puts the Armour on, it attacks immediately.

I assume your party is low level if your looking at Animated Armour, and Dominate Person is a 5th level spell so it might be a bit overkill, as well as the fact that spells require components and you might want to change it a bit I.E. give the PC a saving throw at the beginning of each of it's turns, on a success they regain control and the armour has to use it's action to charm them again, failure means the armour stays in control. As for Hold Person, change Paralyzed to Restrained. Granted I have no idea what the PC could do if they save against the spell, since taking armour off takes time but @RS Conley has a good idea in regards to that.

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To attempt to answer at least a couple of your questions though;

2. I would perhaps rule that the armour takes full damage and the PC only takes half, otherwise it's 3-4 PCs against 1 and there's a good chance you might kill the 1 PC.

3-4. PBH page 144

Armour Proficiency: Anyone can put on a suit of armour or strap a shield to an arm. Only those proficient in the armour’s use know how to wear it effectively, however. Your class gives you proficiency with certain types of armour. If you wear armour that you lack proficiency with, you have disadvantage on any ability check, saving throw, or attack roll that involves Strength or Dexterity, and you can’t cast spells.

and page 146

Weapon Proficiency: Proficiency with a weapon allows you to add your proficiency bonus to the attack roll for any attack you make with that weapon. If you make an attack roll using a weapon with which you lack proficiency, you do not add your proficiency bonus to the attack roll.

The Armour and PC are still two separate creatures and the Armour doesn't have the same proficiencies as the PC so in short use the Animated Armour's stats and actions rather than the PC's. Side note about the Ring of Protection, according to the DM Basic Rules;

Certain magic items require a user to attune to them before their magical properties can be used. Attuning to a magic item requires that you spend a short rest concentrating on it (this can’t be the same short rest used to learn an item’s properties). An item can be attuned to only one creature at a time.

Answer 4

I will do my best but there is a lot here.

Should the Animated Armor's control over the PC be implemented via the grapple mechanic?

That's your call since you are creating the Animated Armor. However I do not think that is a good way to go (see below).

The armor would be able to grapple normally, that is its grapple versus the PC. Then the if the armor wants to move the character it could do so at only half the Animated Armor's speed.

I doubt that either would have advantage per se. There are feats and features that may allow grappling advantage.

The struggle would be on going. The PC would attempt to escape the grapple on each turn. Or so one would assume.

If the grapple mechanic is used to implement this tactic, would it be unfair to grant the Armor the "Damage Transfer" property of the Rug of Smothering?

That is another issue that is totally your call. It's a values judgement, not a mechanical one.

While the Armor is controlling a PC, should the "combined creature" (combination of PC and Armor) attack using the PC's attributes and attacks or the Armor's attributes and attacks?

The armor should not be able to make attacks using the PCs stats. One can assume that the PC would be resisting such attempts.

Using the grappling rules theory, I would say that every time the Animated Armor had an action it would have to either decide to continue grappling the PC, or make an attack.

If it decided to continue to grapple that PC, it couldn't make any other attacks. If it wanted to attack another creature, the Animated Armor, would have disadvantage at the very least, and the PC would automatically escape the grapple.

should the combined creature use the PC's AC or the Armor's AC?

I would consider that to be the Armor's base AC bonus based on type.

The shield bonus would only apply if the character was not resisting the Animated Armor (See above).

Regarding the shield; imagine trying to wield a shield while you have trouble moving your arm the way you want to). This would be the case for both the Animated Armor and the PC.

Even so, if the Animated Armor wanted to use the shield, it would have to posses the appropriate skill.
Does you animated armor know how to use a shield?

The ring of protection would still work though.