A retired demon king's hold on influence while officially stepping back from the throne is such a rich character concept. It's less about legions and more about cultivating an ecosystem where their absence is more strategically valuable than their presence.
One classic method is through a legacy system where their former generals or created institutions now run things, but they maintain loyalty through old debts, personal bonds, or the implied threat of a return. The retired king becomes a symbolic check on the new regime's worst excesses. Think of it like a supernatural retirement where your mere existence prevents a coup because everyone knows you could still end them if they violated the peace you brokered.
Another angle is the transformation into a patron or problem-solver for the underworld's new players. They become the ultimate fixer, the neutral party who brokers deals between warring factions because they have no current territorial ambitions. Their power becomes informational and relational, built on a network of favors and secrets accumulated over millennia. The real authority shifts from commanding armies to controlling the narrative and the flow of critical resources, like ancient magic or interdimensional trade routes no one else can access.