The PCs are chasing their quarry through the city sewers. As a chase scenario, there's a round limit; if the PCs don't catch up to the quarry by the end of the round limit, the villain escapes. Because of this, EP damage and Injuries are not much of a threat here.
Since the PCs are chasing after someone, only one PC needs to reach the goal before the round limit expires in order for the PCs to win. This assumes actually catching the quarry isn't dangerous for the PCs. If it's important that all the PCs make it there (for instance in case of running a combat scenario), add a round to the round limit.
This scenario is listed as both civilization and dungeon setting. As city sewers, you'd be justified running it as either one, or as both. If you run it as both, each entity can choose whether to treat the region as civilization or dungeon at any time, picking whichever is advantageous. For example, a PC could play an Action that can't be played in dungeons by treating the setting as civilization, then turn around and play a Reaction that can only be played in dungeons later. Multiple entities making simultaneous plays can choose a different setting; however, an entity can't change the setting for themselves within a single play.
If that's too complex, just pick the setting that's best for your players, or let them choose one at the start of the scenario.
|Setting||Civilization / Dungeon|
(goal) F - E / \ C D | ^ A > B (start)
|(Region A) The Chase Begins|
|You can hear your quarry splashing through the sewer reservoir to the east; you're not too far behind. The north route might be a shortcut, but perhaps there's a reason your quarry didn't go that way.|
|Down||B||Slimy Wall (Height 2 down to water, no falling damage; can simply jump)|
|(Region B) Sewer Water|
|The leap of faith from above worked out in your favor, at least as much as being largely submerged in sewage can be considered “favor.” There's a ladder climbing out on the north side, but it's in terrible condition.|
|This region requires swimming. Fire-based lighting doesn't work; the darkness penalties may thus stack with the swimming penalties.|
|Entering this region without flight should result in a PC becoming Wet and Filthy. Note that this does not stack with swimming penalties for climbing purposes.|
|North||D||Damaged Staircase (Height 2 up, but no falling damage due to water)|
|West||A||Slimy Wall (Height 2 up, but no falling damage due to water)|
|(Region C) Spider Nest|
|This corridor is thick with spider webs. Their spinners are present, too, and hungry.|
|Vermin Swarm||Respawn: 1|
|(Region D) Maintenance Corridor|
|There is a blessed lack of open water in this stone passageway lined on three sides by pipes, valves, and other strange mechanical devices.|
|The hidden gate controls in this region can close various gates throughout the sewers. PCs that find the controls are assumed to understand them as part of finding them (or, if more appropriate, try something random and get lucky). Gates closing ahead slow down their quarry, forcing backtracking and making it easier for the PCs to catch up.|
|South||B||Damaged Staircase (Height 2 down, but no falling damage due to water)|
|Gate Controls||Hidden: 1d6||If found and activated, increase Round Limit by 1|
|(Region E) Tunnel Junction|
|Three sewer tunnels meet here at this junction. An old door in the west tunnel has been closed and locked; there are signs this was done very recently.|
|West||F||Simple Locked Door|
|(Region F) The Final Leg|
|You catch a glimpse of your quarry just ahead. From the sound of footsteps, it seems exhaustion is settling in; one more push and you'll catch them! These tunnels are filled with vile insects, but you can't let that distract you.|
|East||E||Standard Locked Door|
The PCs have caught up to their quarry. What happens next depends on the details of your setup, though it's generally cleanest not to force the PCs directly into another scenario to force the enemy to comply. The PCs may gain useful information, or be given a reward for bringing in a fugitive.
The enemy gets away. If catching the quarry is critical to advance the adventure, the PCs will need to do some detective work to try to find them; the hunt continues, but it may take days or weeks now. If the PCs were simply chasing someone with some useful information or stolen property, then they lose out on the chance to gain those rewards.
Sometimes chase scenes are more exciting when, instead of a general round limit, there's an incrementing limit as the PCs advance. This can “feel” more like being on the heels of someone (as opposed to, say, trying to escape an area before it collapses). If you want to run this scenario that way, have the PCs start with a 2 round limit. Increase the round limit by 1 the first time a PC makes it into either C or D (but not both; PCs shouldn't get an extra round because they split up). Increase the round limit by 1 again when a PC first makes it into E, then by 1 once more when a PC first enters F. This method is also useful for ending a doomed scenario early if the PCs have bad luck at the beginning.
Region B can be particularly difficult for an ill-equipped party; without climbing equipment or advanced lighting, PCs face two dice level penalties against the Damaged Staircase to escape. That's likely no problem for Delvers or Mariners, but a PC without good Obstacle traversal might be in trouble. Consider adding some grating to the ceiling overhead, allowing in sufficient light from above to illuminate the region. If they're new players, explain what they'll be dealing with if they choose to take that path while they're still in A so they can make an informed decision.
Each round in this scenario is assumed to take somewhere from 2-5 minutes, making it rather quick by exploration standards. If you want a longer timeline, it's best to tweak some descriptions to give the quarry more of a head start (so that PCs are following tracks and trails instead of noises). It could be awkward to try to make sense of the PCs being visibly on the heels of their enemy for 30-60 minutes while dealing with the various Challenges in the scenario. Longer scenarios also mean that one or two PCs might be alone with the quarry for quite some time when they catch up if they were ahead of the rest of the party.
You might want to give the players a choice about what type of scenario to run. You could upgrade the spider swarms to the larger monstrous kind, resulting in a combat scenario if PCs take that route. Since combat is so much faster than exploration in terms of in-game time, you could rule that the PCs gain tons of ground and catch their quarry as long as they're able to best the spiders. If they want to take the longer path avoiding the spiders, then run this scenario, eliminating region C.