Quick post today on a trope not uncommon in fantasy fiction, but which doesn't see much use in gaming (at least, not that I've seen). Allow me to paint a picture for you ...
A powerful wizard appears before a startled group of people and declares that seven of them must at once come with him on an errand of terrible importance. Seven step forward, and once they have grabbed what gear they can, they set off from their safe home and into the wilderness. With the wizard's help, they overcome their first challenge, a small army sent by the Adversary to stop them, but must then part ways with the powerful wizard and sally forth alone.
|High and mid levels in the back, low levels in the front, please|
In games, the adventurers are usually the same level (or close to it), and the accompaniment of a much more powerful NPC under the Referee's control would appear to be a colossal mistake. In fact, it would be if that powerful NPC was to follow along for an entire adventure, getting everyone out of scrapes and leaving little for them to do. As the adventure-starter, though, there are possibilities.
For one thing, the instigator, as we'll call them, can fill the players in on the background of the adventure - the whos and wheres and wherefores.
For another, their presence for the first big challenge of the game permits the Referee to make it a whopper - something epic and un-survivable without the instigator. For a long term campaign, this can be an early shot in the arm of XP for the low level adventurers, to help them on their way. More importantly, it is a way to immerse the players into the setting and the quest in a dramatic way.
Finally, when the instigator leaves, the players will find themselves in a position similar to the conquistadors of Cortes. The adventurers might not be able to turn back, and so they must go forward. The challenges they face from this point on are a bit more keyed to their abilities (though some will be deadly if they are not handled properly), but they will always remember the instigator and their first taste of dangerous adventure.