That one employee that, when he/she/x leaves causes the company to collapse. These so-called white ravens, black swans, or purple squirrels aren’t only a dream to work with; they’ve gathered so much knowledge and expertise in your company that they can’t be missed.
HR departments often (try to) anticipate a catastrophe such as the departure of white ravens by trying to hire exactly the same kind of profiles. Now, signing the first white raven probably involved moving heaven and earth. Let alone finding another one. Moreover, keeping them.
The classic approach to starting from function profiles is rather rigid and tough to scale. Starting from, which hard-skills and soft-skills are required for a job on the other hand, is a leap many HR departments and their companies have to take. When it turns out that these white ravens combine multiple jobs or function profiles within one job, and thus can’t be replaced by one other person, that’s where the biggest challenges are.