You very much have to be flexible. Most valuable would be C and assembly knowledge (and knowledge of the low level details of both is pretty much required). As

You very much have to be flexible. Most valuable would be C and assembly knowledge (and knowledge of the low level details of both is pretty much required). As for tool writing, any high level language (perferably what your employer uses). For example, if MetaSploit is used knowledge of Perl/Ruby would be required. Picking up multiple imperative languages is easy, though. Fortunately most languages in the field are imperative. Depending on the software in question, knowledge of how virtual machines operate can also be useful.