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
- The PC puts on the Armor
- The armor comes to life initiates a grapple check.
- If the PC succeeds, he yells help and we go back to step 2. The PC will get his normal turn.
- 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.
- 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)
- PC can attempt an escape with Athletics only.
- 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.
- 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.
Since the question on whether or not a sprite can remain invisible and use its help action has already been answered, I'll jump to the one bit that hasn't been answered so far:
"My only concern is: is such help strong enough to provide me with advantage?"
Yes you get advantage
The Help action itself is quite remarkable (and underused in my experience): you give up your attack - and thus your ability to do damage yourself - in order to give one of your allies advantage on its next attack on one specific creature you choose.
You give up your Sprite's damage to the foe and in return you get a greater chance to hit, and thus have a bigger chance of doing damage yourself.
The way your sprite's Help action plays out is mostly flavour. What is does, however, is determined by the Help action's definition, which you've already found.
My answer to your question would be: Yes the Sprite would grant you advantage.
But...
You have already stated you're aware of the fact that creatures could sense the fact that your sprite is standing/flying next to them, when you wrote: "(but of course can still be sensed by some creatures, or heard by them)"
Keep in mind that your Sprite has to be within five feet of the enemy in order to take the Help action and can still be attacked while invisible, although this means the attacker will do so with disadvantage, due to the way the Invisible condition is described:
Invisible
An invisible creature is impossible to see without the aid of magic or a Special sense. For the purpose of Hiding, the creature is heavily obscured. The creature’s location can be detected by any noise it makes or any tracks it leaves.
Attack rolls against the creature have disadvantage, and the creature’s Attack rolls have advantage.
If your sprite were to stay in the enemy's reach, this means they could attack it on their turn, but with disadvantage. Your Sprite can, however, fly out of the enemy's reach without provoking an attack of opportunity, assuming it has any movement left.
RAW-wise, an attack of opportunity can only be made against a creature the enemy can see, which gives your Sprite the possibility to aid and move out of the enemy's reach without taking any damage and without requiring the owl's "Flyby" feature.
Edit: I found out this question has already been answered, for more elaborate explanations, go here.
Best Answer
There are several options available to you
Grant advantage against armour
The spell shocking grasp (Player's Handbook p.275) has the clause
This is to simulate how fantasy-electricity \$^1\$ conducts through metal armour to the wearer, approximately negating the armour's protection. You probably want to do something similar.
In past editions (e.g. 3.5e, or its relative Pathfinder), there existed situational variants of AC. The relevant variant here is touch AC, which ignores armour-like bonuses to AC. But 5e has done away with such details for the sake of simplicity. I have seen nothing in D&D 5e that has a different AC calculation in different circumstances. A creature's AC is independent of its state or the type of attack. A creature keeps the same AC even if they are unconscious or paralysed (so cannot move) or targeted by a touch attack (which should ignore armour). Only donning or doffing armour or other protective effects or items change AC.
What has replaced situational variants or modifiers to AC is advantage and disadvantage. When circumstances make a creature easier to hit than its normal AC would suggest, the attack roll has advantage. For example, attackers have advantage against unconscious creatures to approximate their inability to dodge. In a more detailed simulation you would remove their Dexterity bonus to AC (flat-footed AC in 3.5e and Pathfinder), but 5e approximates it with advantage. Likewise, shocking grasp has advantage against opponents in metal armour to approximate the ineffectiveness (or even detriment) of the metal armour.
In your case, you could grant the swarm advantage on attacks against targets wearing medium or heavy armour. Light armour, with +1 or +2 AC, is probably not worth granting advantage against, which normally works out to around +4 or +5 to hit (depending on AC and to-hit), but that's your call. Also your call what to do with shields. But wherever you decide to draw the line, advantage is probably a good way to have an attack which has an easier time hitting armoured targets than their AC suggests.
Use a saving throw instead of an attack
Many spells and other features which only take effect if they touch the target with no regard to armour use a Dexterity saving throw.
For example, disintegrate (PHB p.233) calls for a Dexterity saving throw to avoid the thin green ray created by the spell, presumably to simulate dodging the ray.
Or see the Spawn of Kyuss (Volo's Guide to Monsters p.192), which has a bug related ability.
Again, we a Dexterity saving throw used to simulate dodging something which ignores armour.
A different example is the insect plague spell (PHB p.254). It creates a 20-foot-radius sphere of insects.
There is also the similar infestation spell (Xanathar's Guide to Everything p.158).
For these spells it is assumed there is no opportunity to dodge. Rather, the saving throw represents the target resisting and enduring the biting creatures. Whether you think a Constitution saving throw or a Dexterity saving throw is more appropriate is up to you (and might also depend on whether you already use Constitution saving throws elsewhere in its stat block). Note also that Dexterity saves for half damage are subject to Evasion and the Shield Master feat (but not Dexterity saves for no damage).
Just deal damage, no roll required
A few abilities, like the spell magic missile or a storm giant quintessent's Wind Javelin (Volo p.151), always strike and damage their target, regardless of their AC. These are attack-like in that they use an action and have projectiles. These abilities simulate unerring accuracy, which might not be quite what you are looking for, but it ignores armour.
Other abilities deal damage automatically when creatures are in a certain area, like a balor's Fire Aura, or a fire elemental's Fire Form.
This might be particularly appropriate for your swarm (which normally enter hostile creatures' spaces to attack them anyway), dealing some automatic damage each time it enters another creature's space in a turn. This would represent how it is impossible to escape unscathed when surrounded by the swarm.
\$^1\$ I say fantasy-electricity, because in real life metal armour would conduct electricity around the wearer, protecting them from the shock. This is the principle behind a Faraday cage. The extent of such protection would be dependent on how well connected all the metal parts are to each other and the ground (more metal is better).