Making a side-quest homebrew for my players in an upcoming adventure, pretty stoked for it but having a few points of struggle that could use a braintrust!
Background: there's a new drug on the streets of the city, and a local unit of the city guard is getting paid to provide cover for the gang. There's a boss (the city guard's ward captain), two sub-bosses, and then just thugs/corrupt city guards. The players can disrupt the drug supply and take down the gang sub-boss, which will trigger the corrupt city guard sub-boss to come for them.
What I'm struggling with is how to then hook in the ward captain - the final boss of this one-shot. So far I have it like this
1. If the PCs keep the drugs when they sack the supplier, that triggers the sub-boss to come looking for them. If they don't go after the ward captain soon enough I just send more corrupt guards/thugs their way as random encounters or get them arrested and have to do a jailbreak/boss kill.
2. If the PCs destroy the drugs, same thing as above basically. They get brought in for questioning – meet the head boss and sub-boss – as a pretense to rough them up and later pin the drug trade on them.
3. If the PCs give the drugs to another city guard leader or local authority, they will task the PCs to take down the corrupt city guards (seems a bit of a dodge though? Instead of arresting the corrupt guards themselves).
4. BUT – what if the PCs just don't do anything? Like, assuming they get this far, and disrupt the drug supply, what if they just leave the drugs there? I have it planned that in that case the drugs "disappear" quickly but not sure how to bring in the corrupt city guard action. I'm trying to make it so that the corrupt guards are kind of "aware" of the PCs investigating, and gave them some brutal flavor for how the corrupt guards deal with anyone who even might be aware of their involvement, but not sure on this point.
ALSO: any suggestions for not-lame evidentiary links that could connect the boss to the drug supply? We're a bunch of lawyers, and even if the guards are corrupt the PCs will need some kind of evidence to justify their actions to other city guards and city leaders (who will be rightly concerned about some of their own being killed, regardless of the corruption). Given how careful I've made the corrupt city guards in their covering up their involvement I don't imagine the boss having a paper trail.
If they don't do anything I would have the corrupt guards see them as a scapegoat. So obviously the town knows about the drug and an arrest could lead to increased morale for the city. So the party must defend their reputation all the while uncovering how far up the chain the corruption goes.
Considering how careful the top guys are the party may need to catch them in the act. Maybe an npc who is suspicious of the guards could gift them a magic item for some sort of recording.
Hm, so basically just treat it like my #2 up there and force an interaction that way. That could work (assuming the PCs don't throw some other wrench my way!).
Not a bad idea about a magical recording device, or like a friendly Kenku or something maybe. I was trying to think of something more clever than "oh look a letter with the ward captain's print on it." Very much "The Wire" and I like that! I'd only need a hook of some way to get them to think of that, and maybe make it that they have to sit on it until the right person or magical item comes their way.
Any additional suggestions for how to get the corrupt drug boss "on record" as it were? What I have so far is that following the PCs disrupting the drug supply, an informant lets a friendly NPC know that the gang supplying the drug and smugglers for it are going to meet with corrupt city guard boss, ostensibly to discuss retaliation and recovery of their operation. Somehow the PCs need to get that meeting recorded so that it's not hearsay, but I don't want to just add new mini-bosses at this stage like a new gang lieutenant to replace the one that the PCs killed earlier – so I'm thinking that they get told by the friendly NPC not to go in for a smash.
At the same time, this is their story to play out too! So I have to check myself against scripting it too heavily as I try to prepare for what they may do.
Will they have interacted with the Ward Captain before? Perhaps he's the one who has sent them to find the gang, and he instead sends them to a variety of dangerous situations which are intended to kill them, but don't because "they's the heroes", and when they return from a second or third fruitless search of a cave which actually contained bears, or an abandoned warehouse which was suddenly full of bandit assassins, they return to see the Warden meeting with a drug lord, or they hear raised voices in the wardens office, and overhear him explaining how they will "no longer be a problem".
Alternatively, you could have them break into the drug warehouse, and find that the people they see are actually just scarecrows. As soon as they realise, the city guard break down the door and arrest them for making and distributing drugs. They then have to either evade capture or escape from jail, then find a way to prove their innocence or the Ward Captain's guilt.
These are clever ideas...the set-up especially, as it would force them to go into hiding and could make a compelling act in the story arc. This is all occurring with the Waterdeep: Dragon Heist main story so I'm also trying to keep some of those NPCs relevant like Jalester, but the set-up could work if the PCs are at a sufficient level maybe in reputation that they feel they can reach out to him secretly for help?
I want to make it so there are at least 2 ways to connect as evidence the ward captain to the drug network – physical evidence, some magic item that the gang mini-boss would have that is paired with something the ward captain has, and gathered evidence (like a Kenku informant who could recount the ward captain's statements, or some magic item recording device). Still struggling a bit with the second part, but fortunately this one-shot probably won't happen for 3 months easy.
If they decide not to do anything with the drugs, for example just walk away after the fight, you could have one of them see a sign of movement down an alley or on a rooftop, maybe to indicate that they were being watched. Then a few things could happen. 1. Someone plants the drugs in their room and they get arrested (or escape arrest) or 2. They get charged for the crimes/drugs. Either way, it the ward captain could be the one charging them and ramrodding a shoddy case through the system.
I like that idea – basically give them a chance to get on their guard but likely set up the "set up". This being Waterdeep and the Code Legal punishment for killing guards being death, I'm working the angle that the corrupt city watch captain and his enforcers (who may or may not be backed by a Masked Lord in some meta-game against another Masked Lord) enjoy a lot of political, legal, and social protections that make them perfect for covering a the drug trade connection.
This ties back to the issue where, ultimately, the PCs need to prove the Ward Captain's guilt or their innocence (for having killed city watchers, even if they were corrupt). In a nutshell this one shot/side quest looks like this right now:
Prologue: I just flavor Waterdeep with users for this new drug, PCs can hear rumors about it, but nothing that really would capture their intrigue for being in Waterdeep. Drug has a chance to kill users so bodies are slowly piling up.
Act 1: the hook, someone like in a tavern attracts the PCs attention and speaks a little too loudly about how some of the city watch are allowing this trade to happen. The corrupt city watchers have made their incredibly brutal reputation known so most denizens don't say anything about the city watch (a very high insight + persuasion before would give some clues). A spy for the corrupt watch hears the NPC and leaves the tavern, prompting some dice rolls for perception and stealth if the PCs follow. In any event, the person's body is delivered to the PCs dismembered the next day - as a warning.
Act 2: if the PCs do nothing, I'll have a friendly NPC warn them that they're being watched and need to tread lightly or even quit the ward altogether for a while. If PCs ask a friendly NPC for more information and succeed on a lower persuasion check than before the friendly NPC will connect the murder to a corrupt city watch lieutenant, and name drop the ward captain as probably behind it. If PCs continue to do nothing I might let this story just die – if the PCs want to investigate, they can get information from a user on who their dealer is. The PCs can then go confront the dealer and try to get information out of them after a chase and fight. The dealer tells them where the drugs are coming into the city, and the corrupt city watch provide cover and squash rivals in exchange for a hefty bribe. The dealer works for the gang but never met any of the city watch, as that's handled between the sub-bosses. If the PCs don't kill the dealer, the corrupt city watch lieutenant does so publicly when the PCs are somewhere like a public square (and brutally, to fit a theme). PCs later get a note letting them know they're getting no more warnings to quit.
[At this point I presume the players have some kinda pull in the city, otherwise the corrupt city watch would just kill them]
Act 3: PCs go disrupt the supply chain, either on their own or at the direction of, say, a friendly NPC like Jalester from the main campaign (who doesn't believe the city watch are involved, without evidence). This leads to needing to find, and surveil, the drug storage and seeing the city watch lieutenant and gang lieutenant exchange words and gold. The PCs fight the gang lieutenant and some mobs in the warehouse, where if successful they have the (current stock) of undelivered drugs and a clue connecting the gang to the ward captain (a magic amulet or badge that is paired to one the ward captain has, but the PCs don't know that).
At this point, PCs have a chance for random encounters with gang assassins, and maybe corrupt city watchers?
THIS is where I started to ask for help from you all - with thanks! To scout out what to do with their potential actions with the drugs.
Act 4: PCs need to connect the gang-drug operation with the corrupt city watch, and the city watch lieutenant sub-boss tries to kill them as the main event for this act. Depending on what they do in Act 3 with the drugs they might have been arrested and met the ward captain at this point, too, who is equally brutal as the lieutenant but is also a paladin (oathbreaker, I imagine). This is maybe the hardest Act for me to think about as it requires the players to get evidence to connect to the ward captain or their arrest/escape if they're fugitives wanted for the murder of city watchers. Once the city watch sub-boss is killed I think they're boned so far as the city watch is concerned until the end of the quest, except for maybe Jalester or any PC connections.
So far the ideas I have here are 1) get a DnD-flavored wiretap on the ward captain or the lieutenant and pick up a discussion with a gang agent about how they'll kill the PCs and reestablish things, 2) get a Kenku informant to help them, with some kind of meeting between the gang and the ward captain [we're lawyers so this should also be the most fun in some ways]. The trouble here is the ward captain would never allow a Kenku near such a meeting - I'm playing him as a mastermind in keeping up appearances and eliminating paper trails/witnesses. So need to ponder this more.
Alternatively, as mentioned, somewhere in this act could be a set-up where PCs are told about a meeting, or another drug shipment, and go there only to find the corrupt city watchers, lieutenant, and maybe the ward captain? I could have the fight there, but would need the ward captain to leave or survive somehow (and in either case, the PCs are then fugitives). The best evidentiary link I can think of here is that the ward captain mentions to the PCs the amulet that he gave to the gang sub-boss before, and show his own amulet which had been magically paired. This would then provide proof – along with a zone of truth on the PCs or the ward captain I suppose – of the connection. I'm treating the money here as super fungible and easy to embezzle or whatever so they can't go the financial angle.
In any event, the end of Act 4 is either 1) they have evidence and are authorized to assault the corrupt city watch captain, rather than have other city watch do it so that the watch can try and see who might have been behind the ward captain, or 2) the PCs are fugitives and will be forced to have a trial with a city magister in the end.
Act 5: Final boss fight – a basic confrontation with the ward captain and some of his elites in his lair of sorts. Success results in the the amulet (physical evidence), if they didn't have the aural evidence from a wiretap or the Kenku.
Act 6: If they were fugitives, after killing the boss the city watch storm in and arrest them all, leading to a trial with a magister which should all be roleplay and no actual combat (persuasion and insight checks and all that).
This all seems a little forced to me - I think this has a chance to go sideways as there are a lot of things that have to happen, and if they don't, the plot goes awry. So how to keep that from happening...
Are any members of the group Rogues with a tie to the Guild in Waterdeep? Possible hook if so - the Guild is not getting paid for this new drug and it is undercutting their profits.
Does the party have a base in Waterdeep? Like a tavern that they have gone to, NPCs they know, etc? If so, have something happen to one of the NPCs. Maybe they see one of the guards give/sell the NPC something (the drug) and later that NPC is either dead or very sick. The PCs may actually end up saving the NPCs life and find out about this drug.
Make the way the drug is killing people resemble something the party has an interest in. My party is on a kick against undead, so I could get their interest by making the drug kill people in a way that resembles having blood drained by a vampire, minus bit marks.
What magical items do you party members have that would enable them to spy things out? This could be a thieve's dream assignment - getting blackmail goods on the city watch?
That's excellent feedback, thanks! My worry is that there's a degree of coercion to this vs making it organic, so seeding in ways to make it connect to their play styles and other objectives is valuable. I'm thinking to have them just base themselves in Yawning Portal – so ample opportunity to have NPCs around that can help trigger clues or events. Making it part of a rogue-thief PC's story might be really valuable as well; I think I want to be prepared for whatever the PCs decide to do, but not prescriptive for their choices.
Where I'd like to be is have a framework for the side-quest/one shot set up, so that by the time they can do anything with it they might have some ties or other flavorings I could incorporate. Right now we're starting fresh at level 1 for all of them, and this seems like a level 3-4 or even level 5 thing to be doing.
Making the drug kill in a way that fits their interests is also a good idea. I was rolling with having the drug confer a damage for certain rolls and benefits for others (weighted for damage though). Like, if they consume it they have to make a save, and failure to make the save results in a lot of psychic damage based on how far away from the DC they rolled; i.e. I set it to DC10, and a PC rolls a 9, they get 1d10 psychic damage while an 8 would be 2d10 psychic damage (whereas success might give them some temporary boon, like advantage on their next save throw, followed by a point of exhaustion). This can be retooled as need be, and maybe the properties of the drug require a PC to commit time to doing medicinal study on it.
The only magic item I can find from googling that would work is from 3.5 – the Arcane Recorder. For 5e, what I thought I'd do is have it that this very powerful item exists but is treasured and held by the Commander of the Watch, the Lord Mage, or Open Lord, and modified from 3.5e it's less powerful but still potent (the diamond orbs have to be physically planted and retrieved, and their functioning time is reduced from 20 hours to, like, 10 or 5 even). Using the Recorder also requires an intelligence save from the user, who if they fail is auto-knocked unconscious or even killed outright (requiring death saves or another PC to help them). So the PCs would either 1) have to get the Arcane Recorder, either by persuading to borrow it from the person who has it, or 2) steal it, which would be incredibly difficult but could be fun if any/all the PCs are thief-y, and then 3) successfully plant an orb on the suspected ward captain or someone in the gang who might meet with the ward captain, 4) retrieve the orb, and 5) survive the process of using the Recorder to witness the contents of the orb.
The Kenku being the other way to get "on record" any verbal exchanges, I figured the PCs could find a friendly Kenku NPC acting as an informant who's main task would then be to hear the ward captain discuss their role in the drug trade. The challenge there being getting the Kenku nearby the ward captain, who would certainly not want one nearby him, and keeping the Kenku safe during and after it witnessed any exchange.
The backdrop of a set-up + going underground to then prove their innocence/the ward captain's guilt is really captivating too, which might make for a more interesting Act 3 or Act 4.
My party came across a stone of sending - or one half of it. They tried to see if they could activate it and use it to listen in on the other half, which I thought was pretty clever... Also, they have a hat of disguise which is a thief's dream...
Making a side-quest homebrew for my players in an upcoming adventure, pretty stoked for it but having a few points of struggle that could use a braintrust!
Background: there's a new drug on the streets of the city, and a local unit of the city guard is getting paid to provide cover for the gang. There's a boss (the city guard's ward captain), two sub-bosses, and then just thugs/corrupt city guards. The players can disrupt the drug supply and take down the gang sub-boss, which will trigger the corrupt city guard sub-boss to come for them.
What I'm struggling with is how to then hook in the ward captain - the final boss of this one-shot. So far I have it like this
1. If the PCs keep the drugs when they sack the supplier, that triggers the sub-boss to come looking for them. If they don't go after the ward captain soon enough I just send more corrupt guards/thugs their way as random encounters or get them arrested and have to do a jailbreak/boss kill.
2. If the PCs destroy the drugs, same thing as above basically. They get brought in for questioning – meet the head boss and sub-boss – as a pretense to rough them up and later pin the drug trade on them.
3. If the PCs give the drugs to another city guard leader or local authority, they will task the PCs to take down the corrupt city guards (seems a bit of a dodge though? Instead of arresting the corrupt guards themselves).
4. BUT – what if the PCs just don't do anything? Like, assuming they get this far, and disrupt the drug supply, what if they just leave the drugs there? I have it planned that in that case the drugs "disappear" quickly but not sure how to bring in the corrupt city guard action. I'm trying to make it so that the corrupt guards are kind of "aware" of the PCs investigating, and gave them some brutal flavor for how the corrupt guards deal with anyone who even might be aware of their involvement, but not sure on this point.
ALSO: any suggestions for not-lame evidentiary links that could connect the boss to the drug supply? We're a bunch of lawyers, and even if the guards are corrupt the PCs will need some kind of evidence to justify their actions to other city guards and city leaders (who will be rightly concerned about some of their own being killed, regardless of the corruption). Given how careful I've made the corrupt city guards in their covering up their involvement I don't imagine the boss having a paper trail.
If they don't do anything I would have the corrupt guards see them as a scapegoat. So obviously the town knows about the drug and an arrest could lead to increased morale for the city. So the party must defend their reputation all the while uncovering how far up the chain the corruption goes.
Considering how careful the top guys are the party may need to catch them in the act. Maybe an npc who is suspicious of the guards could gift them a magic item for some sort of recording.
Damien Steinrich
Hm, so basically just treat it like my #2 up there and force an interaction that way. That could work (assuming the PCs don't throw some other wrench my way!).
Not a bad idea about a magical recording device, or like a friendly Kenku or something maybe. I was trying to think of something more clever than "oh look a letter with the ward captain's print on it." Very much "The Wire" and I like that! I'd only need a hook of some way to get them to think of that, and maybe make it that they have to sit on it until the right person or magical item comes their way.
Any additional suggestions for how to get the corrupt drug boss "on record" as it were? What I have so far is that following the PCs disrupting the drug supply, an informant lets a friendly NPC know that the gang supplying the drug and smugglers for it are going to meet with corrupt city guard boss, ostensibly to discuss retaliation and recovery of their operation. Somehow the PCs need to get that meeting recorded so that it's not hearsay, but I don't want to just add new mini-bosses at this stage like a new gang lieutenant to replace the one that the PCs killed earlier – so I'm thinking that they get told by the friendly NPC not to go in for a smash.
At the same time, this is their story to play out too! So I have to check myself against scripting it too heavily as I try to prepare for what they may do.
Will they have interacted with the Ward Captain before? Perhaps he's the one who has sent them to find the gang, and he instead sends them to a variety of dangerous situations which are intended to kill them, but don't because "they's the heroes", and when they return from a second or third fruitless search of a cave which actually contained bears, or an abandoned warehouse which was suddenly full of bandit assassins, they return to see the Warden meeting with a drug lord, or they hear raised voices in the wardens office, and overhear him explaining how they will "no longer be a problem".
Alternatively, you could have them break into the drug warehouse, and find that the people they see are actually just scarecrows. As soon as they realise, the city guard break down the door and arrest them for making and distributing drugs. They then have to either evade capture or escape from jail, then find a way to prove their innocence or the Ward Captain's guilt.
Make your Artificer work with any other class with 174 Multiclassing Feats for your Artificer Multiclass Character!
DM's Guild Releases on This Thread Or check them all out on DMs Guild!
DrivethruRPG Releases on This Thread - latest release: My Character is a Werewolf: balanced rules for Lycanthropy!
I have started discussing/reviewing 3rd party D&D content on Substack - stay tuned for semi-regular posts!
These are clever ideas...the set-up especially, as it would force them to go into hiding and could make a compelling act in the story arc. This is all occurring with the Waterdeep: Dragon Heist main story so I'm also trying to keep some of those NPCs relevant like Jalester, but the set-up could work if the PCs are at a sufficient level maybe in reputation that they feel they can reach out to him secretly for help?
I want to make it so there are at least 2 ways to connect as evidence the ward captain to the drug network – physical evidence, some magic item that the gang mini-boss would have that is paired with something the ward captain has, and gathered evidence (like a Kenku informant who could recount the ward captain's statements, or some magic item recording device). Still struggling a bit with the second part, but fortunately this one-shot probably won't happen for 3 months easy.
If they decide not to do anything with the drugs, for example just walk away after the fight, you could have one of them see a sign of movement down an alley or on a rooftop, maybe to indicate that they were being watched. Then a few things could happen. 1. Someone plants the drugs in their room and they get arrested (or escape arrest) or 2. They get charged for the crimes/drugs. Either way, it the ward captain could be the one charging them and ramrodding a shoddy case through the system.
I like that idea – basically give them a chance to get on their guard but likely set up the "set up". This being Waterdeep and the Code Legal punishment for killing guards being death, I'm working the angle that the corrupt city watch captain and his enforcers (who may or may not be backed by a Masked Lord in some meta-game against another Masked Lord) enjoy a lot of political, legal, and social protections that make them perfect for covering a the drug trade connection.
This ties back to the issue where, ultimately, the PCs need to prove the Ward Captain's guilt or their innocence (for having killed city watchers, even if they were corrupt). In a nutshell this one shot/side quest looks like this right now:
Prologue: I just flavor Waterdeep with users for this new drug, PCs can hear rumors about it, but nothing that really would capture their intrigue for being in Waterdeep. Drug has a chance to kill users so bodies are slowly piling up.
Act 1: the hook, someone like in a tavern attracts the PCs attention and speaks a little too loudly about how some of the city watch are allowing this trade to happen. The corrupt city watchers have made their incredibly brutal reputation known so most denizens don't say anything about the city watch (a very high insight + persuasion before would give some clues). A spy for the corrupt watch hears the NPC and leaves the tavern, prompting some dice rolls for perception and stealth if the PCs follow. In any event, the person's body is delivered to the PCs dismembered the next day - as a warning.
Act 2: if the PCs do nothing, I'll have a friendly NPC warn them that they're being watched and need to tread lightly or even quit the ward altogether for a while. If PCs ask a friendly NPC for more information and succeed on a lower persuasion check than before the friendly NPC will connect the murder to a corrupt city watch lieutenant, and name drop the ward captain as probably behind it. If PCs continue to do nothing I might let this story just die – if the PCs want to investigate, they can get information from a user on who their dealer is. The PCs can then go confront the dealer and try to get information out of them after a chase and fight. The dealer tells them where the drugs are coming into the city, and the corrupt city watch provide cover and squash rivals in exchange for a hefty bribe. The dealer works for the gang but never met any of the city watch, as that's handled between the sub-bosses. If the PCs don't kill the dealer, the corrupt city watch lieutenant does so publicly when the PCs are somewhere like a public square (and brutally, to fit a theme). PCs later get a note letting them know they're getting no more warnings to quit.
[At this point I presume the players have some kinda pull in the city, otherwise the corrupt city watch would just kill them]
Act 3: PCs go disrupt the supply chain, either on their own or at the direction of, say, a friendly NPC like Jalester from the main campaign (who doesn't believe the city watch are involved, without evidence). This leads to needing to find, and surveil, the drug storage and seeing the city watch lieutenant and gang lieutenant exchange words and gold. The PCs fight the gang lieutenant and some mobs in the warehouse, where if successful they have the (current stock) of undelivered drugs and a clue connecting the gang to the ward captain (a magic amulet or badge that is paired to one the ward captain has, but the PCs don't know that).
At this point, PCs have a chance for random encounters with gang assassins, and maybe corrupt city watchers?
THIS is where I started to ask for help from you all - with thanks! To scout out what to do with their potential actions with the drugs.
Act 4: PCs need to connect the gang-drug operation with the corrupt city watch, and the city watch lieutenant sub-boss tries to kill them as the main event for this act. Depending on what they do in Act 3 with the drugs they might have been arrested and met the ward captain at this point, too, who is equally brutal as the lieutenant but is also a paladin (oathbreaker, I imagine). This is maybe the hardest Act for me to think about as it requires the players to get evidence to connect to the ward captain or their arrest/escape if they're fugitives wanted for the murder of city watchers. Once the city watch sub-boss is killed I think they're boned so far as the city watch is concerned until the end of the quest, except for maybe Jalester or any PC connections.
So far the ideas I have here are 1) get a DnD-flavored wiretap on the ward captain or the lieutenant and pick up a discussion with a gang agent about how they'll kill the PCs and reestablish things, 2) get a Kenku informant to help them, with some kind of meeting between the gang and the ward captain [we're lawyers so this should also be the most fun in some ways]. The trouble here is the ward captain would never allow a Kenku near such a meeting - I'm playing him as a mastermind in keeping up appearances and eliminating paper trails/witnesses. So need to ponder this more.
Alternatively, as mentioned, somewhere in this act could be a set-up where PCs are told about a meeting, or another drug shipment, and go there only to find the corrupt city watchers, lieutenant, and maybe the ward captain? I could have the fight there, but would need the ward captain to leave or survive somehow (and in either case, the PCs are then fugitives). The best evidentiary link I can think of here is that the ward captain mentions to the PCs the amulet that he gave to the gang sub-boss before, and show his own amulet which had been magically paired. This would then provide proof – along with a zone of truth on the PCs or the ward captain I suppose – of the connection. I'm treating the money here as super fungible and easy to embezzle or whatever so they can't go the financial angle.
In any event, the end of Act 4 is either 1) they have evidence and are authorized to assault the corrupt city watch captain, rather than have other city watch do it so that the watch can try and see who might have been behind the ward captain, or 2) the PCs are fugitives and will be forced to have a trial with a city magister in the end.
Act 5: Final boss fight – a basic confrontation with the ward captain and some of his elites in his lair of sorts. Success results in the the amulet (physical evidence), if they didn't have the aural evidence from a wiretap or the Kenku.
Act 6: If they were fugitives, after killing the boss the city watch storm in and arrest them all, leading to a trial with a magister which should all be roleplay and no actual combat (persuasion and insight checks and all that).
Thoughts? Improvements? Especially in Act 4.
This all seems a little forced to me - I think this has a chance to go sideways as there are a lot of things that have to happen, and if they don't, the plot goes awry. So how to keep that from happening...
Are any members of the group Rogues with a tie to the Guild in Waterdeep? Possible hook if so - the Guild is not getting paid for this new drug and it is undercutting their profits.
Does the party have a base in Waterdeep? Like a tavern that they have gone to, NPCs they know, etc? If so, have something happen to one of the NPCs. Maybe they see one of the guards give/sell the NPC something (the drug) and later that NPC is either dead or very sick. The PCs may actually end up saving the NPCs life and find out about this drug.
Make the way the drug is killing people resemble something the party has an interest in. My party is on a kick against undead, so I could get their interest by making the drug kill people in a way that resembles having blood drained by a vampire, minus bit marks.
What magical items do you party members have that would enable them to spy things out? This could be a thieve's dream assignment - getting blackmail goods on the city watch?
That's excellent feedback, thanks! My worry is that there's a degree of coercion to this vs making it organic, so seeding in ways to make it connect to their play styles and other objectives is valuable. I'm thinking to have them just base themselves in Yawning Portal – so ample opportunity to have NPCs around that can help trigger clues or events. Making it part of a rogue-thief PC's story might be really valuable as well; I think I want to be prepared for whatever the PCs decide to do, but not prescriptive for their choices.
Where I'd like to be is have a framework for the side-quest/one shot set up, so that by the time they can do anything with it they might have some ties or other flavorings I could incorporate. Right now we're starting fresh at level 1 for all of them, and this seems like a level 3-4 or even level 5 thing to be doing.
Making the drug kill in a way that fits their interests is also a good idea. I was rolling with having the drug confer a damage for certain rolls and benefits for others (weighted for damage though). Like, if they consume it they have to make a save, and failure to make the save results in a lot of psychic damage based on how far away from the DC they rolled; i.e. I set it to DC10, and a PC rolls a 9, they get 1d10 psychic damage while an 8 would be 2d10 psychic damage (whereas success might give them some temporary boon, like advantage on their next save throw, followed by a point of exhaustion). This can be retooled as need be, and maybe the properties of the drug require a PC to commit time to doing medicinal study on it.
The only magic item I can find from googling that would work is from 3.5 – the Arcane Recorder. For 5e, what I thought I'd do is have it that this very powerful item exists but is treasured and held by the Commander of the Watch, the Lord Mage, or Open Lord, and modified from 3.5e it's less powerful but still potent (the diamond orbs have to be physically planted and retrieved, and their functioning time is reduced from 20 hours to, like, 10 or 5 even). Using the Recorder also requires an intelligence save from the user, who if they fail is auto-knocked unconscious or even killed outright (requiring death saves or another PC to help them). So the PCs would either 1) have to get the Arcane Recorder, either by persuading to borrow it from the person who has it, or 2) steal it, which would be incredibly difficult but could be fun if any/all the PCs are thief-y, and then 3) successfully plant an orb on the suspected ward captain or someone in the gang who might meet with the ward captain, 4) retrieve the orb, and 5) survive the process of using the Recorder to witness the contents of the orb.
The Kenku being the other way to get "on record" any verbal exchanges, I figured the PCs could find a friendly Kenku NPC acting as an informant who's main task would then be to hear the ward captain discuss their role in the drug trade. The challenge there being getting the Kenku nearby the ward captain, who would certainly not want one nearby him, and keeping the Kenku safe during and after it witnessed any exchange.
The backdrop of a set-up + going underground to then prove their innocence/the ward captain's guilt is really captivating too, which might make for a more interesting Act 3 or Act 4.
My party came across a stone of sending - or one half of it. They tried to see if they could activate it and use it to listen in on the other half, which I thought was pretty clever... Also, they have a hat of disguise which is a thief's dream...