Your combat multiplier is an icon which appears underneath your health and mana bars on the top of the left hand side of your screen whenever you fight someone. It is an indication of how good you are doing at fighting either by using will, ranged or melee weapons and when you do battle, this icon shows up and the number increases. If you use lightning at a high level, you can target more than one enemy at a time and get a crazy high combat multiplier (this is what I do coz I'm rubbish at fighting!). Also, the more enemies to kill, the longer you have to increase your multiplier.
This probably makes little sense and someone is bound to correct me/elaborate accordingly - but I hope you have a better idea of what it is now.
Oh, and welcome to the forums!
