Magic items with caster dependent effects

Question

There are a number of spells and abilities with effects that depend upon the factors outside the level of the caster or the level of the spell as cast.

Most commonly, the D.C. of a spell check can vary based upon the casters spell ability. Somewhat less common is something like metamagic that allows you to do something x times, where x is the ability bonus of the caster. Often the cost of doing so is effectively the same as increasing the level of the spell

If an item was enchanted with a spell cast at a higher slot (ring of level 5 magic missile) we would expect it to behave as the higher level spell (and have 5 darts). Hypothetically, one would expect an item imbued with a metamagic'd spell to operate the same way (ring of Ennervated Eldritch Blast). When a magic item possesses one of these variable metamagic factors, do we use the relevant ability score of the creator/caster or those of the user?

Answer 1

Magic items are assumed use the minimum ability score necessary to meet the prerequisites for their creation. That is, according to Magic Items, while…

A creator can create an item at a lower caster level than her own, but never lower than the minimum level needed to cast the needed spell. For example, a 15th-level wizard could craft a wand of fireball at 10th caster level, or even as low as 5th level (the minimum caster level for fireball, a 3rd-level spell), but no lower. (Dungeon Master's Guide 282)

…usually a magic item doesn't need an ability score except to determine the saving throw DC needed to resist its effects:

For a saving throw against a spell or spell-like effect from a magic item, the DC is 10 + the level of the spell or effect + the ability modifier of the minimum ability score needed to cast that level of spell. For example, a 2nd-level spell’s save DC would be 10 + 2 (for the spell being 2nd level) + 1 (for needing at least a 12 in the relevant ability score to cast a 2nd-level spell), or a total of 13. (214)

(These quotations—sans examples—are also in the SRD on Creating Magic Items and on Saving Throws against Magic Item Powers, respectively) Of course, there exist exceptions: staffs use the wielder's ability scores, this DM gravitates toward using the higher of the ability score necessary to create the intelligent magic item and the intelligent magic item's own ability scores, and some magic items indicate an ability score because they must like the shield of Prator (DMG 282) (major artifact; 15 lbs.).¹

The only time typical magic items use their creators' ability scores instead of this default is if a magic item's creator possessed the feat Enhance Item, the benefit of which is

Choose any item creation feat you already know. When you create an item with that feat, adjust the DC for saving throws required by the magic item, if any, by your key ability modifier. (Epic Level Handbook 114)

While it appears in the Epic Level Handbook, the feat Enhance Item doesn't possess the epic type. It's only prerequisite is another item creation feat, but, as indicated, the feat only applies to one item creation the creature already possesses. The feat Enhance Item can be taken multiple times, though, once for each item creation feat. Despite the feat's restrictions, it's still one of the game's most powerful feats, often ignored even in optimization circles because so rare is the DM that allows the feat Enhance Item into his campaign.

So far as I'm aware, in official Dungeons and Dragons 3.5 material there are no metamagic feats that are limited in their uses by—or otherwise affected by—the metamagic feat's possessor's ability scores.

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¹ Untouched by the 3.5 revision, the shield of Prator (or, to eliminate product identity, the shield of the sun) remains described in the Dungeon Master's Guide (2003) as a 3e-style large shield. It's a heavy steel shield according to the premium edition Dungeon Master's Guide (2012).

Answer 2

You'd use the minimum required

The magic items section says:

Magic items produce spells or spell-like effects. For a saving throw against a spell or spell-like effect from a magic item, the DC is 10 + the level of the spell or effect + the ability modifier of the minimum ability score needed to cast that level of spell.

So if a scroll of a spell that allows the use of 1 bolt per point of [casting stat mod] were used, the minimum stat required to cast that spell would be used, rather than either the creator's or the user's.

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Also, crafting a metamagic version of a spell into a scroll increases the save DC if the spell level increase from the metamagic is high enough so that the spell level required to cast the spell needs a casting stat that has a higher bonus.

Unless that metamagic spell were heightened, however, the higher spell level would not itself increase the save DC, as the spell level doesn't actually change.

This still means that items use the minimum casting stat required, rather than the creator's stat, to produce the spell effect.