What are you calculating? 

EDIT: You can use calculate to derive the value, and use <Duration>, but I don't think it calculates each turn.

Well, I know that you can't calculate to Duration. You can only use calculate to derive Value, MinValue, and MaxValue. I've been trying to think of a way to make the first part work, but without having a variable for the round (or not knowing what it is), I'm not sure you can do it.

Thanks for the help guys. I'll give the poisonshot ability a look today. If I can get it to work, I'll post the xml as a reference for others.

The question is if he can do a spell that does different things in each subsequent round. You guys are confusing the issue.