As to the original story question, perhaps something along these lines:
* The warlock manages to summon the Old One, who in turn opens portals that begin vomiting smaller monstrosities.
* The police are helpless and are all but wiped out.
* The military is called in. Tanks seems to be capably of damaging the smaller stuff, but with hideous and unsustainable losses.
* The government, flailing for a solution, authorizes a nuclear strike. Just before the missile hits, a dome of roiling energy several miles across materializes. Everything outside of the dome is flattened by the nuke, but when the dust clears there is a perfectly circular section of the city completely intact.
* The players, at a loss as to how to stop the madness, are somehow able to learn that the summoning ritual involved tying together the essences of the Old One and the warlock (possibly as a bid by the warlock to control the Old One). This is the weakness they need.
* The players fight their way through the lesser outsiders, create a diversion to seperate the Old One from the warlock, then attack and manage to destroy the Warlock in an epic battle.
* The Old One makes an appearance a moment too late ... but to all appearances is unaffected by the warlock's death. (It has been significantly weakened, it's grip on existence in the real world is slipping, however.)
* By this time, a second (massive) nuclear strike is en route. The players are still no match for the creature, but manage to distract it just enough that the nuclear strike gets through. Thanks to the players' actions, the beast's body is vaporized, it's essence banished back to it's home plane.
The players have been destroyed, but their sacrifice has saved the world (well, except for the city). The End.
Or is it? Perhaps a few of the lesser outsiders managed to escape the blast, either by getting far enough away, or by burrowing. Perhaps this will plant the seeds for the next campaign...
