That sounds like something you could even do in a macro, but I haven't played with macros much yet. But the API can do this easily. Can you describe the exact process, are the d6 and d4 used to look something up on a table or just extra to hit/damage? Because the API could do more than just roll another die. The user would type in the chat window something like !jdice. The script would then roll a d20, if its a 1 or a 20 it rolls another set of dice. Then it outputs the results to chat window. It can say critical hit or miss, show the numbers, anything you like. If someone else doesn't get to this before tonight I'll write something up. But get me the exact mechanics of what you want done. Is this a game system I could read up on or just something you custom do? The script can do anything (almost) you want it to so may as well do as much as it can for you instead of just an extra roll, unless that simply all you want. :)