Every business has someone who knows how things really work. That person is valuable. The dependence built around that person is not. Leaders often mistake institutional knowledge for employee excellence, avoiding the question of why ordinary work cannot continue without private memory and improvised judgment.
Documentation alone does not solve the problem. A thick procedure manual can be as useless as no manual if it describes the ideal process rather than the real one. Start with a dependency interview, not a request to write everything down. Ask the key employee what people contact them about, which decisions wait for them, and which failures they prevent without reporting.
Then test the process without the expert in the room. Give a trained colleague a realistic scenario and watch where the work stops. A process is not transferred when a document exists. It is transferred when another qualified person can produce a reliable result.
An indispensable employee should be indispensable because of the value they create next, not because the business cannot reproduce what they already know.