Summons that have Instinctive Effects
Your players should choose (or you could create) summons with Instinctive Effects. The are actions which trigger of the summon hasn't been told to do anything by the end of the player's turn.
For example, the Druid Level 1 daily, Summon Giant Toad has:
Instinctive Effect: If you haven’t given the toad any commands by the end of your turn, it attacks the same creature that it attacked during your previous turn. If it can’t do that, it attacks an enemy within 3 squares of it if it can. Otherwise, it moves its speed to a square within 3 squares of the nearest enemy.
Be sure to check out this question (Is there any way to get multiple standard actions for Summoners in D&D 4E?) for other related suggestions.
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.
Best Answer
You are right, the stat block is a bit weird. I'd suggest using the trap as presented in the Dungeon Master's Guide. It is also the current version in DDI, so it is easily arguable to be the "correct" version.
Here are a few of the things from the original that were changed (for the worse IMHO).
As for XP, you do not need to destroy the sphere to gain the XP. The XP is gained for successfully completing the encounter. Usually that will mean getting by or past the sphere. See the first sentence of the Traps and Hazards section in the Dungeon Master's Book: