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In Codex Necrons the court can join other units and become part of that unit permanently because the rule says it can join a specific unit. If a Daemon Prince is given the "Prophet of the Voices" it states he can only join units of possessed from Codex Crimson Slaughter. So does that mean he is allowed to now?

He still needs permission to join units. All Prophet does is restrict what units an IC can join.

The rules for the necron royal court are a smidge more complex than that, notably they also include specific instructions on how to add crypteks or lords to units and how they work within that context.

In this case being able to only join squads of possessed doesn't help much if the model is unable to join anything due to it's inherent rules. It would either need the independent character rule or some similar bypass, none if which is granted by 'Prophet of Voices' to my awareness.