So, recently, I had an issue where I had to make a call on a custom-made weapon, and I'm not sure I handled it very well. The player was using a human alchemist, and instead of throwing 'bombs' every turn, he came to me with a concept for a weapon. Basically, it's a bunch of vials of acid, alchemist fire, and the like, wrapped up together in a fine-mesh net, attached to a staff with rope, and wielded in a similar fashion to a mace.
My response: if you take the appropriate penalties for wielding it, then you can use it. It came out to something like a -6 on attack roles (-4 for non-proficient, -2 for using what amounted to a large-sized mace). However, as he stated, so long as it actually struck the target the glass vials would break and deal damage, so he had to hit against their touch AC.
I thought the idea was impressive, and didn't think it would actually be a huge problem for an enemy… until it was. He ended up tossing something like 200 vials in the net, and managed to kill something a full 10 CR higher than the group, on his first turn.
Well, as you can guess, he's not allowed to use that item anymore. However, I really enjoy seeing players put the effort into this kind of creativity, and I would hate to discourage it just because their end-goal is to make something broken. (After all, if it wasn't beneficial to them in some way, why put in the extra effort?)
So, how do I handle situations like this? How do I reward him for his inventiveness, while not allowing him to break the game? Advice on this specific situation would be appreciated, but mainly I'm looking for a way to handle the issue as a whole. Any experience with the same problem is welcome, of course.
Best Answer
Encouraging inventiveness is easy. You've already done it, in fact: You simply have to reward it by allowing it to (occasionally) succeed.
Avoiding breaking the game is also easy: You simply disallow anything that lets players to bypass the obstacles presented by the game with less effort than it would take to tackle those obstacles in a more traditional way.
Obviously, these two goals are hard to reconcile, but I can suggest a few things that might help.
First, let your players have the benefit of their invention, but remember to account for the costs: In your example, your alchemist has developed a devastating weapon, but one which eats through very expensive ammunition at a disturbingly rapid rate. Once the disparity between party member wealth levels grows a bit, he might start to worry. Also, given that it is a devastating weapon, even if it is expensive, ask yourself why other people don't use it. I imagine it would take just one orc with the Improved Sunder feat to make your player realise that there are pitfalls involved in waving acid vials over his own head.
Second, remember to include at least a few situations in which the invention does not work, to keep your players on their toes (and encourage future inventions): There's no end of creatures immune to acid and fire, so let them crop up occasionally - and your alchemist would be in strife if away from civilisation for an extended period.
Finally, don't be afraid to condtradict previous rulings if you can come up with a good reason for it. Yes, you should avoid it most of the time, but inventions are a special case, since by definition they're trying something new and unknown: Invention is inextricably paired with discovery. Be tactful, and start by saying "There's something I didn't think of that's kind of relevant to that idea you came up with," but be firm, and try and provide a reasonable in-game justification for it. "Your acid net? The ropes that make it up only have about two hit points, so it should be disintegrating every time you use it. Also, how have you been protecting yourself from the splash?"