I have offered to take a turn at running a game for our group. I have a story that I think they will find interesting, mechanics to cope with the fact that some players sometimes just don't show up sometimes and access to the normal GMs vast library of books (honestly, I have never seen so many, not even in the local games shop). I also have spent time designing unique loot items that can be used to advance the plot (and backup plans for if the party don't think of that).
The story calls for the party to face an vast army of usually annoyingly easy creatures. Narratively it would make sense to have a few thousand goblin archers supported by mounted units, casters and fighters in equally unfair numbers. Said army would show up at a key point in the story to prevent the players from accessing the really cool loot until later. This would be the first clue that someone is really ticked off at the party. The most sensible choice would be for the party to leg it and hold up in the nearby castle where the plot can unfold while the castle is surrounded (some politics some hidden room exploration etc).
The problem is I know the party and they might very well try and fight the entire damn army. How do I handle combat if they choose to not run away? I don't fancy roiling initiative that many times.
I considered saying "The sky blackens with arrows which rain down upon your characters. Despite your best efforts you suffer massive damage. Please roll up new characters." But that really would not be kind, or very fair – the party might survive long enough to realize that running is a good idea.
Best Answer
Luckily, Pathfinder has rules for this already. It is called the troop monster subtype. It's similar to a swarm, but different and designed for exactly this case, where you pit PCs against large groups of soldier types. (It's separate from their mass combat system, which does not incorporate PCs well).
They developed it for their Reign of Winter AP, here's the blog post explaining it more and providing a troop of Russian riflemen.
As the troop does automatic damage each round to all the PCs, they should get the point and head into the castle after several rounds of that.
They're pretty similar to some quickie mass combat rules I came up with for the same goal (main differences are mine roll to hit and are also easier to construct from component creatures), which is seamless integration of PCs with mass combat with much-reduced rolling. I've used these with my group for a long time, and they fit the niche perfectly - PCs sometimes will go head to head against a unit but are always re-surprised at how hard they hit.