There's a global threat even the mightiest armies of the realm cannot defeat

>There's a global threat even the mightiest armies of the realm cannot defeat
>But four plucky teenagers can
How do you justify this in your seting? Personally I always find dungeon crawls devolving into wargames where PCs are basically Hero units.

Congratulations, you have discovered the thing tabletop RPGs (or at least dnd) were literally designed to do.

Plucky teenagers have ATTITUDE, the most important force in the universe.

Killing a dragon that's been murdering kingdoms for most of recorded history tends to go to ones head a bit.

/thread

Genre convention. Maybe the heroes have something special about them, maybe they're just the right people in the right place at the right time. Either way, they can do what the army cannot, because they're the damn heroes.

That isn't to say that the army isn't also doing something useful elsewhere, but if the army could just solve the problems then you'd be playing a completely different kind of game.

Gods are child molesting creeps, those chosen few had "divine favor" bestowed upon them.
Attempt to dissect chosen one, extract the favor and implant it into more worthy individual were not successful, so they have to be used as they are.

If you are talking about singular threat think of it like about tank in under medieval setting.
None of soldiers can hope to penetrate it's armor but a group of four teens with AT-gun can.

If global threat is head of the army then four heroes are spec-ops that assassinate enemy leader so his army of darkness breaks without his leadership.
Sneaking entire army is hard, 4 people not so much.

My dnd group doesn't save the world in our dungeon crawl.

I never do 'save the world' plots.