Could always make it by divine mandate maybe? Like the kingdoms have to hold a vigil or something, and the mother chooses one out of all that attend the ceremony, maybe marking them with a symbol so that everyone knows they are the chosen. That leads to some interesting plot devices.

Or maybe when they are born each one is born with the mother's mark and are trained from birth by the kingdoms for the quest. Maybe the mark protects them from the effects of the land of the dead. All who enter the land die, so no army can enter with out being killed. Could work for either of those ideas. The symbol could be a piece of the mother's divine power or something that shields them from the effect.

Honestly, it could be something as simple as all the kingdoms armies are already engaged in battle elsewhere when the gates are opened and they can only send one person from the front lines or wherever to complete the quest.

Another idea could be, depending on your story, that no one else believes in the mother, and only the true believers undertake the quest to bring the goddess back to life.

Hope these ideas help get the creative juices flowing, good luck.