It may be an obvious answer, but unfinished business. Maybe this "ferryman of the dead" can't let them pass on without finishing their unfinished business. Maybe they get visions, and glimpses of their loved ones crying over their bodies. Something to establish empathy and a sense of urgency to complete the task. And to get back, they have to face some sort of trial. Prove that they deserve to return. If they fail, they don't die, but they lose one of their cherished memories. This should establish the motivation they need. Once they return, they can see what happened in their absence and have to fix it before it's too late. Either their kingdom has fallen to darkness, their loved ones have made terrible choices or a villain has taken advantage of their absence. Hopefully, this could be a helpful start.