Tannat

The cause of death was cape cobra venom, but the book has the victim ingesting it rather than having it injected.  Unless there’s something special about cape cobras, as far as I can tell, ingesting snake venom is not a reliable way to kill someone since to actually be effective there generally needs to be a cut or something in the mouth that would allow the venom to pass into the blood stream.  Of course, ingesting snake venom still isn't a good idea, but it probably won't be fatal.