Thanks for the reply...
its not only the hellfire blast, I've got a spell pack installed and it provides a nice number of dots (damage over time), and how do I increase the damage dealt by these invocations ?
You edit the scripts, generally the includes referenced at the top of each script. For warlocks everything is in nw_i0_invocatns.nss, then after you do that you compile each spell script that uses that.