A slightly vague heading, but what we are talking about here are agencies who basically offer background consulting services to put their client in the most informed position for whatever advantage.
Advertised services will include: investigation, intelligence, risk mitigation, litigation & claims support, investigative due diligence, commercial & operational trouble shooting, crisis management, good governance, private security. These will be in support of client businesses, multi-national businesses, law firms, investment banks, governments, international agencies and even family or private clients.
In a corporate due diligence sense, the services will be offered for the clients' advantage in operation and development, helping to evaluate and enable strategic opportunities, assessing and minimizing exposure to strategic risk. This might include advice on the commercial, geographic or political considerations involved in expansion into new sectors and jurisdictions, recommendations on commercial strategy, decision-making and risk mitigation, specialist due diligence on potential business partners, governance and regulatory issues, introduction and engagement with policy makers and stakeholders in new regions.
In a more litigious vein, there might be the tracing of assets, lifting corporate veils, cross border fraud and money laundering investigations, digital forensic services and internal investigations.
Some of this type of investigative work will be more mundane than it sounds, and you are also probably going to have to man the marketing pumps in a similar fashion to a private practice solicitor, but a legal background will offer many transferable skills, for example in presentation, analysis, reporting, advices, risk management, finding devil in the detail etc.. Corporate lawyers, litigators and advocates will probably feel in familiar territory, or at least be au fait with the overall purpose.
Consultants can be generalists, a 'G3' maybe, or industry specialists, such as 'Grey Page' servicing maritime clients, or more in the vein of private security and risk management, but where plenty of academic analysis works in tandem with military know-how; 'Aegis' for example.