For bonus fun, use baleful polymorph to turn someone into a tiny animal. Use animal messenger to send the animal to an enemy with a message that reads, “I prepared explosive runes today.” Boom.
As for your question...
Does this sound like a good ruling for the spell, or is there some kind of cheese still present with it that we haven't considered? I want the spell to be effective, but not overpowering, because I believe in the Gentleman's Agreement. Also, what do you think of this ruling for the spell?
Let us consider the spell in question
- PLAYER’S HANDBOOK (3.0)
- PLAYER’S HANDBOOK 1 (3.5)
Abjuration [Force]
Level: Court Herald 3, Magewright 3, Rune Domain 4, Sorcerer/Wizard 3
Target: One touched object weighing no more than 10 lb.
You trace these mystic runes upon a book, map, scroll, or similar object bearing written information.
Here's your problem.
The target line says one thing, the text says another. Now, as a DM, I believe the intent is not to write the explosive runes, but to have writing that you turn into explosive runes. So, can you write on a wall then cast this spell? Well, technically, no. The wall is over ten pounds. A kind DM might say the brick is seperate from the wall, but by the rules, there are no bricks, just 10x10 foot sections of wall.
Now, how does this apply to your arrows? Well, First of all, we must determine if you are correct in that you can simply disrupt the writing and detonate the spell.
Another creature can remove them with a successful dispel magic or erase spell, but attempting to dispel or erase the runes and failing to do so triggers the explosion.
So, any attempt to erase the words would trigger the explosion, unless the person doing it is a thief and makes a disable device roll. Destroying the words by breaking the object it is on is a valid way to attempt to "erase" something, so this is actually right on the money.
Now, can you put writing on an arrow? Certainly. Mystical runes of power are written on magic items all the time. The arrow is under 10 pounds. It should work, no problem. Nothing in the spell indicates the font size, so technically, if you had monks write the entire bible on a grain of rice, you could put explosive runes on it.
Now here's the tricky part, does breaking the arrow count as "erase". That's your gray area.
Generally speaking, ammunition that hits its target is destroyed or rendered useless, while normal ammunition that misses has a 50% chance of being destroyed or lost.
Destroyed certainly counts for an attempt to erase something, but rendered useless? The arrow could be bent, the feathers sheered off, or just the tip could be stuck in someone and the shaft turns out to be fine. It's that last part that causes me some concern. I would suggest then that there is a 50% chance that the arrow is destroyed and 50% rendered useless. If it's rendered useless, it is still readable, and thus the runes do not detonate. That said, if the bad guy acts all macho and snaps the arrow off while it's stuck in him, well... Boom.
As for adding it to flasks, tough call. "Do Not Drink: External Use Only" certainly counts as writing. Here's the problem that I see.
I throw the flask. The flask breaks. The Explosive runes explode. The fluid takes 6d6 points of damage. The fluid has exactly 1 hit point. The poison/acid/holy water/alchemist's fire is destroyed in the blast and never has time to connect with the target.
Now, throwing JUST the flask should have the desired effect, and be a ranged touch attack as well, but I see no means putting anything inside and having it take effect. The explosive runes simply go off first. Now, if you wanted to slather up your flask with contact poison, (Not injury, the flask would never do damage as a ranged touch attack) then it being on the outside of the runes, it would take effect first, then the explosion.
Now, if you had something that exploded, say... a black powder Bomb or something like that, I would say explosives would stack, but beside that crap from eberron, I know of no explosive devices that is "standard"
I hope that answers your question and I welcome counter arguments, since I plan on adding this to the explosive runes entry of The SpellBook.