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.
Spoony, from the Counter Monkey blog series, mentions Alchemist's Fire every now and then. It's great, and everyone thinks it's great, but here's the thing: eventually, you're going to roll a 1. Eventually, the party's pyromaniac is going to set someone next to them on fire, and then be lit on fire themselves, if they didn't do that to begin with. And then what happens when the person loaded up with alchemist's fire is on fire? They explode. And who's right next to them? The entire party.
Someone swinging around 20 vials of alchemist's fire on a pole is no better.
Wield chemicals with extreme caution
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.
Here's how you do it: let him use it. The pyromaniac is carrying around at least 200 vials of acid, alchemist's fire and other dangerous and possibly volatile chemicals, since you said he managed to use all those in one blow in one encounter. All of those containers are weak enough they'll shatter if smashed into someone. That's a problem.
What if someone runs up to the pyromaniac, swings a club, and smashes his backpack - the one full of vials of acid and alchemist's fire - whilst he's wearing it? Oh, the club didn't pass his AC? That means he either dodged it or his armor blocked it - maybe his backpack stopped the blow.
What if the character misses with the weapon, and the chemicals swing around to smash against the shaft itself? What if, even after normal use, the staff has acid trickling down it?
What about the day he rolls a 1 - which will happen one day - and swings the mace into an ally within the weapon's reach? What if he swings it back and smashes it on the wall behind himself?
What if, even out of battle, he trips over, falls, and smashes his hundreds of vials of chemicals? What if he fails a climbing check, or tumbles after a dodgy jump check?
Even if he hits, those chemicals are going to splash. On other people.
Pyromaniacs have a short life expectancy, and so do the people who travel with one. If your player wants to walk around with dozens of vials of every dangerous chemical he can think of, he and his party should be aware of the dangers.
Because this is a dangerous situation, you should talk to your players in advance about what should happen when the user of the weapon hits, misses, or rolls a dreaded 1. That way, when 20 vials of alchemist's fire and acid one day explode above the pyromaniac's head and leave no recognisable remains after a roll of 1, they won't be annoyed that it was a GM fiat - everyone knew that if a 1 was rolled, that was one of the possible outcomes. The rest of the party will just be relieved it wasn't one of them getting hit.
Target his weaknesses
So: your pyromaniac has thought ahead and has fire and acid resistance - from resist energy, I assume. His party members might still be entirely vulnerable - but let's assume they're protected too (via communal resist energy - which would only last a few minutes).
Even with protection, your pyromaniac and his weapon is still quite vulnerable. I suggest you take advantage of it.
The first vulnerability is that resist energy can only resist so much damage, so unless his caster level is high enough to exceed the damage cap for the weapon, he and his allies can still take damage from a disastrous swing.
The second, and greater one, is his enemies' ingenuity. No doubt word would spread about such a fearsome and eccentric weapon. Any organised group of enemies should begin strategising how to weaken him. At this point, you've reached the situation a lot of new weapons create: an arms war, with one side inventing weaponry, and the other working out how to counter it or make it work against its wielder. Here's some potential strategies for countering his weapon:
- Target his weapon when it's loaded. Arrows and rocks (from a sling or hand) can smash those vials before they see any proper use. Best case scenario: target it whilst the end loaded with vials is over a party member's head, or right near someone.
- Target his bag of holding. Find a way to puncture, slash, or burn a hole in it. Even if he protects it, it must be vulnerable when he's loading his weapon.
- Force him to pop his resist energy ahead of time, so that he can't use it when he wants to use his weapon. Then bombard him with energy he's not resisting: electricity, cold or sonic damage. After he's used it up, and resist energy's effects have ended, ambush him with fire.
- Don't let him swing the weapon safely. Don't give him enough room, and have enemies readied with a chain to trip him in mid-swing, or a bow or sling to smash the weapon.
If your players aren't up against any enemies with the organisation or ingenuity required to engage in this arms war, then either make the enemies begin to get organised or introduce organised or clever enemies. Either that, or forfeit the arms war and leave enemies with no recourse against a devastating weapon - or disallow the weapon.
Ultimately the goal is to keep things in line
If your character invents a powerful weapon, make it come with a cost. Luckily, a pyromaniac's weapons generally come with an inherent cost. If he works out a way around those costs (as he has), make the weapon difficult to use effectively, instead of subjecting helpless enemies to a super-weapon.
The point of all this is to just keep the reward and the risk or cost in proportion. The point is also to prevent your party from wielding a super-weapon nobody can defend against.
All these actions work under the assumption you allow the weapon at all, and ruling it out is certainly a simpler option, but that won't encourage ingenuity (something I love to encourage in my own games).