Name a role after the person, and make it an honor to fill that role
Certain individuals take on so many jobs, and become so important to the project that when they leave, the project is in more than just serious trouble. Name a role after the person, and make it an honor to fill that role. People will want to emulate the legendary person, and do just as good a job.
On occasion there is one person who knows everything, does everything, for a project. This is a person of exceptional expertise, but the project is at risk if this person leaves. It seems like it would take at least two people to fill his role, yet there may be no easy way to split the work owing to the new communications paths and formalisms that would be needed. Anyone asked to fill the role would need extensive training, ideally from the ‘legend’ that originally fills the role.
Legend roles tend to fade away after a time, as the work gets spread out among other roles. This is a good thing; it helps keep a ModerateTruckNumber. Having a legend role in the first place is an indication that there is a need for more skills transfer (see LackOfExpertise for some approaches). The rapid expansion of the IT market over the last 3 decades of last century meant that expertise was always in short supply. With slower expansion and modern approaches to skills transfer, it may be reasonable to hope that legend roles become a thing of the past. Where many are expert, nobody becomes a legend.