Shrink Item and the Oversized Starknife

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Shrink Item is a very interesting spell. It enables you to reduce an item to a tiny fraction of its original size, making it easy to transport. It also enables you to bring the item back to its original size via command word or tossing it on a solid surface:

You are able to shrink one non-magical item (if it is within the size limit) to 1/16 of its normal size in each dimension (to about 1/4,000 the original volume and mass). This change effectively reduces the object’s size by four categories. Optionally, you can also change its now shrunken composition to a cloth-like one. Objects changed by a shrink item spell can be returned to normal composition and size merely by tossing them onto any solid surface or by a word of command from the original caster. Even a burning fire and its fuel can be shrunk by this spell. Restoring the shrunken object to its normal size and composition ends the spell.

One could think about using it for arrows or similar, but this detail on Reduce Person seems to block that usage:

Melee and projectile weapons deal less damage. Other magical properties are not affected by this spell. Any reduced item that leaves the reduced creature's possession (including a projectile or thrown weapon) instantly returns to its normal size. This means that thrown weapons deal their normal damage (projectiles deal damage based on the size of the weapon that fired them).

However, this clause there:

"This means that thrown weapons deal their normal damage"

Seems to indicate that, if a throw weapon is resized during flight to its original size, it will deal damage based on that original size, not on the reduced one. This has curios the implication that the momentum of such resized weapons is changed according to the size, increasing alongside the mass and volume of the throw object.

With that in mind,

Suppose that one creates a massive starknife, as a custom magic item, enchanted in such manner that its wielder can activate or deactivate a shrink effect using a swift action. When shrunk, this weapon is around the size of a normal starknife. When expanded, it becomes four thousand times larger.

The idea is that the wielder of this weapon throws this thing and uses a swift action to enlarge it during midfligth, impacting its target as a gigantic steel star.

Is it possible to make this work, or is this too dependent on DM fiat to pull off?

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Note: If this is possible to make work with 3.5 material, that would be welcome as an answer too.

Answer 1

What the question asks is entirely subject to GM fiat because new magic items can't enter a campaign without GM approval. I mean, a reader can speculate what a new magic item might do, but, ultimately, it's the GM who allows—or doesn't allow—into her campaign a player's own new magic item.

What this means is that without GM involvement the spell shrink item targets only a nonmagical object, in which case the spell does what it says it does. Applying the spell permanency to the spell shrink item allows only the permanency spell's caster to control the object's size, typically taking a standard action on the controller's part. A PC can brew an oil of shrink item, craft a wand of shrink item, scribe a scroll of shrink item, and use other item creation feats that use language like You can create a magic item of the appropriate type of any spell you know to make magic items of shrink item. GM approval must be sought for other uses of the shrink item spell, like the effect the spell shrink item has when it's a prerequisite for a new magic item. (The shrink item spell's mandate may have been (ahem) enlarged by other rules, but I'm unaware of them, and they're likely irrelevant here unless those rules apply directly to creating magic items.)

Consider a different approach

Starting with a spell and speculating as to what could happen when the spell's used in new ways is certainly one way to approach the creation of, for example, a new specific magic weapon. However, there's an alternative: Starting with the new magic item then speculating as to its prerequisites and, especially, its price. Magic Item Gold Piece Values alludes to this alternative method: "The correct way to price an item is by comparing its abilities [n.b. not its prerequisites!] to similar items…, and only if there are no similar items should you use the pricing formulas to determine an approximate price for the item." (Also you may be interested in this question and this question.)

I offer this alternative approach, first, because, as previously stated, the GM must approve any new magic item that a player develops anyway. Further, even if a player gets the GM's take on how something works in her campaign, that's not, like, permission for the player to introduce to the campaign new game elements without the GM's approval! For example, no matter how the player thinks the spell shrink item works, and no matter what the GM said about how the spell shrink item works in her campaign, it's still the actual magic item—that neutron starknife itself—that's the important thing, and that neutron starknife must have the GM's approval before it can be a part of that GM's campaign. Really, a GM won't (or, at least, shouldn't) pre-approve new magic items simply because the player wants the magic item to possess as a prerequisite a specific spell! That's a recipe for disaster.

If you don't have a GM right now but you still want to create new magic items, I urge writing a formal magic item description as per Magic Item Descriptions for whatever new magic item develops from the speculation—be it a specific magic weapon like the neutron starknife or a magic weapon special ability like hurled embiggening or something else entirely. Then, if unsure, post those descriptions on this site or elsewhere and ask questions about those new magic items like Is the price of this new magic item appropriate?, Are these reasonable prerequisites for this new magic item?, and, especially, Is this new magic item balanced in comparison with other similar game elements? (The site really can't know how a GM will rule, but the site is pretty good at assessing new game elements.)

Then, when you join a campaign and your PC goes to create these new magic items, you'll already have feedback on your new magic items. This still won't guarantee that a GM will approve for her campaign the new magic items, but the feedback you received should offer suggestions that will make the new magic items more likely to meet with a GM's approval.

Answer 2

Much too DM dependent:

• Shrink item only comes undone on striking a solid surface—not a target—or via command word. Command words are usually standard actions. You need DM approval for a custom shrink item effect whose command word is quicker than that.

• You need DM approval to attach your custom shrink item property to a weapon.

• You need DM approval to take a swift action between releasing your starknife, and it striking a target. This is not clearly allowed by the rules: while swift actions are like free actions, and free actions can happen in the middle of other actions, such as between attacks in a full-attack, it’s not clear that between the release of the starknife and it striking is a valid point in time to take a free action—if there even is a point in time between those two events.

At this point, you might as well just ask your DM for a special magic starknife that deals damage as if it was Colossal, and call it a day. Because that’s effectively what you’re asking for, and were it me, I’d be pretty annoyed if I was asked

• “Can I have an effect that uses a swift-action command word?”

• “Can I take a swift action between releasing a thrown weapon and it striking?”

and found out that what I was really being asked was

• “Can I have a weapon that deals damage as if it was Colossal?”

I mean, I wouldn’t let you get away with it—I would ask what you were doing—but if somehow you did manage to blindside me with that I’d be pretty annoyed, and for that matter I’d not really appreciate the attempt even if I “caught you.”

Answer 3

Yes, with some GM lenience

Let's say that your GM is lenient and does allow such an item, which they don't have to do: it's common practice for GMs to just ban all custom magic items to prevent a lot of overpowered stuff from existing. If you don't want to frustrate your GM and look like a rules lawyer in its worst sense, tell your GM in advance what you are trying to perform.

You need to determine the activation method to reduce your Colossal Star Knife to the Medium size and enlarge it back to Colossal. It does not have to be command word, actually. Many magic weapons are activated when you simply use them as normal (e.g. you swing your sword, you shoot a bow). You can make it so that your dagger does not even require a Swift action, but is rather enlarged by merely tossing it with an normal attack.

Your GM may agree that it's OK to reduce it back to its normal size on command word, or that you make it small again by simply attempting to grab it (Use-activated again).

Then, ask yourself: is it OK to effectively pay 30.000 gold (Spell Level (3) * Caster Level (5) * 2000) for +11 points of damage effectively added to the item? Is it overpowered or underpowered? Well, looks like another question worth asking if it interests you.