What effect will allowing Spell Storing on Arrows/Bolt have?

Question

Context

I'm currently running a homebrew pathfinder campaign. The players are in the middle of adventure called "Mako's Missing Tome". Mako is a powerful wizard that the PCs owed a favour. Unbeknownst to the players, BBEG hired a thieves guild to steal Mako's "Tome of Inventions". As payment for the favour the PCs were tasked to retrieve the book from the thieves guild's hideout and return it to him.

In my group I have a wizard who has already expressed his plans to examine the book and attempt to craft whatever magic items reside within. I also have a Bard who focuses on ranged combat with a crossbow. As well as a Barbarian, Druid and Paladin but they are less relevant. All characters are currently level 4, they will be going to level 5 at the end of this adventure. This is a long running campaign expected to reach at least level 20.

Criteria

My question concerns the magic item that the book contains. It must meet a few criteria:

• Must be a Magical Weapon of some sort, something that would interest the BBEG enough to bother getting it stolen
• Must be rare and inventive, Mako is rather eccentric so nothing too commonplace
• Must be expensive/difficult to create, otherwise others would have made it already

Context given in case this is a X-Y question. But mainly I wanted to ask if there are any issues with the item I'm considering.

Item

My idea would be that Mako has found a way to add the spell storing ability to ranged ammunition.

Spell Storing

A spell storing weapon allows a spellcaster to store a single targeted spell of up to 3rd level in the weapon. ... This special ability can only be placed on melee weapons.

I also found this forum and this forum that discuss the issue but come to vastly different conclusions. Should the correct price be 8,355gp per 50 arrows or SLxCLx50 per arrow?

What effect will allowing this in my game have? My hope is that it will funnel some of the Wizard/Druid's spellcasting ability into the Bard who sometimes struggles to be effective in combat. Are there other effects that I should be concerned about before adding this?

Answer 1

Spell-storing ammunition sounds cool, but there's definitely balance problems to address, especially at high levels and with players who are good optimisers. The two below are what immediately sprang to mind, but others will probably think of more things that need to be considered.

Cost

Firstly, the obvious sensible way to use this property is to make a bunch of ammunition and store a lot of different spells in different bolts ahead of time. A standard craft would give you 50 pieces of ammunition, and then a few days of prep before you head off on adventure gives you potentially up to 50 uses of third level spells. We'll assume the use of the normal crafting rules for magical items, so the working out of the cost at 8355gp/50 is correct - the alternate working you found seems to be quoting from a Dragon magazine article which gives rules for making ammunition with a specific spell imbued into it, which isn't quite what we're talking about.

50 3rd-level scrolls would cost near to 20,000gp; 8,355gp for the bolts is considerably cheaper. A wand casting a 3rd level spell with 50 charges is around 12,000gp, so closer, but still more costly and considerably less versatile, since you only get to choose one spell. Prep time is basically free, so this makes the bolts a much more cost-effective and attractive option for accumulating the use of offensive spells than either scrolls or wands.

You could go some way to mitigating this difference by making spell-storing on ammunition a +2, rather than +1, property. Then a set of 50 bolts would cost 18,355gp, which is pretty much in line with a similarly sized set of scrolls. You might want to make them even more expensive than that, since it's much easier for most characters to fire a bow or crossbow than it is to use a scroll.

Action Economy

Secondly, action economy. It's normally quite difficult to cast several spells in a single round. However, most characters will have the ability to make multiple attacks with a full attack action, certainly by the time you're able to craft items like these; if a fighter with a bow can launch 3 or 4 attacks a round, and each of those attacks can also be some kind of spell, they may even be slinging spells faster than the rest of the party put together. The damage potential gets kind of immense if you've got damaging spells saved in those arrows, or you could be forcing enemies to make several save-or-sucks in a single round.

With the melee version there's no way to use one weapon to drop a spell on someone more than once a round, so ideally some kind of way to make the ranged version also limited to a single spell per round is a good idea. If I were designing it, I might declare that the spell-storing ammunition needs to be primed somehow with a swift action before it is launched, otherwise it just acts like an ordinary +1 (or whatever) bolt. That way, you'd only effectively be able to use one such piece of ammunition per round, which is much more in keeping with normal action economy.