Can the Immovable Object spell target an object held or worn by an opponent?

Question

The description of the immovable object spell says (Explorer's Guide to Wildemount, p. 187):

You touch an object that weighs no more than 10 pounds and cause it to become magically fixed in place. [...]

Can immovable object target a piece of armor or the weapon of a target creature?

For example, can I touch a guard's helmet and make it immovable, in order to restrain him? If so, is there a save, or is it a simple spell attack?

Answer 1

You can't freely touch an object worn by your enemy

The immovable object spell description has no attack roll involved. You always cast it successfully.

This is consistent with other non-combat spells:

You touch a closed door, window, gate, chest, or other entryway, and it becomes locked for the duration (arcane lock)

A flame, equivalent in brightness to a torch, springs forth from an object that you touch (continual flame)

On the contrary, combat spells (by "combat spells" I mean spells which are intended to be used in combat in order to directly harm or hinder an opponent) usually have some kind of attack roll or saving throw:

Your touch inflicts disease. Make a melee spell attack against a creature within your reach (contagion)

Normally you can choose an object worn by your enemy unless the spell description says otherwise. However, if a spell requires you to touch an object, you must have an access to this object in order to touch it. Being able to freely touch the object is a requirement, not a "fluff". You can't freely touch a sword your enemy is actively swinging.

It's the same reason why you can't manipulate object worn or carried by your enemies — for instance, we assume you can't push a button on an immovable rod your enemy is carrying, regardless of the fact there is no "this item must not be worn or carried" phrase in the magic item description.

The spell description will probably be fixed

The spell description is ambiguous in its current state. It should be clarified in the upcoming errata one way or another:

• If you can't target worn or carried objects, this should be said explicitly in order to avoid confusion
• If you can target worn or carried objects, and there actually was the intent behind the spell, the spell description must include mechanical consequences of such an application. Can you fight with a immovable sword, if you don't move away more than 10 ft? The spell description doesn't answer this in its current state.

Your DM will probably ban this spell in its current state

Your DM will probably either ban the spell completely (as Aaron3468 did), or rationalize you can't freely touch worn/carried objects (as I did). Otherwise the spell becomes super-effective (read "too overpowered for its low level") exclusively as a combat spell, which I believe wasn't the designer's intent.

Answer 2

Quick Summary:

1. Can it target armor or weapons? Yes. Armor: only shields and Leather or Padded. Weapons: All aside from Pikes and Heavy Crossbows.
2. Does it restrain the wearer? Sometimes, except it doesn't cause the restrained condition.
3. Is there a save? Yes, for every 10 feet the affected item will be moved. It also takes an action to perform the save.

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Immovable Object says that you touch an object that weighs no more than 10 pounds, creatures you designate ... can move the object normally, and you can set a password which suppresses this spell for 1 minute. The object can hold up to 4,000 pounds of weight and a creature can use an action to make a Strength check against your spell save DC to move the object up to 10 feet.

This means that you can restrain guards, causing them to require a Strength saving throw for every 10 feet they intend to move while wearing the item. They can however use a Use An Object special action once on each turn to interact with their helmet. Assuming it has no strap or means of securing it to their head, in most cases they would be able to escape without any real consequence.

As far as Immovable Object goes with armor: only Shields, Leather, and Padded Leather armor can be affected. All other armor listed in PHB is heavier than 10 pounds. Mechanically, the opponent's weapons are the most efficient targets. Depriving your opponent of a powerful weapon attack option can reduce them to unarmed, natural, or improvised weapons.

The spell afflicts its own condition on the target object (and in some rare cases, the user of that object). It does not cause the restrained condition unless your GM makes a houserule.

So this spell's value is ultimately determined by your GM. It's very similar to illusion magic, since your mileage will vary.

For example, one of the GMs in a homebrewing community rules that Bag of Devouring requires some sleight of hand checks when players attempt to use it during combat since there aren't specific combat rules for sticking arms into bags. This spell, however has a range of Touch that permits it to operate during combat (Shocking Grasp would also not work if Touch ranges were disallowed, for example.).

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There's some reasonable incredulity at the spell's strength. Yes, it is strong (and can be cast at higher levels). The cantrip Light is a good reference for adjudication, as the spell's text contains the same bug that otherwise exists in weapon buffs such as Holy Weapon which don't explicitly prevent opponent's objects from being targetted. Mileage varies because your GM will make calls appropriate to the game.