There is a large town turning into a small city that mainly grew via farming. They were very close with nature, initially including druids in their ruling council. It was exceptional, as the local druid circle was mostly an elven one that typically would keep to themselves but did start sharing some of their wisdom with the farmers. However, as the town grew, they became more industrious and trade started taking over. In fact, their relations with the local druid circle have strained to the point that there is little contact between the citizens and the druids.
Quality and quantity of crops have gone down since the days of druids in the council. The Trademaster accused the druids of secretly doing this, so that the town needs them. There were some subtle clues although it was never proven. Recently though, the quality have gone down significantly and the farmers thought of appealing the druids. The Trademaster hates this, saying she’s proven right, that the city would be at the druids’ hands if they did that, she wants the town to be self-reliant.
In fact, she has a good idea about how to improve the quality of crops: Ankhegs. And that’s why she hires an adventuring party. She does look for someone outside the town because she thinks citizens’ thoughts on the issues are too clouded with the nostalgia of the past. She asks the party to clear the area of Ankhegs, which she reckons there are about 6 adults. She insist the party to take the tunnels as it’s the only way to ensure there are no adults left. It doesn’t look like the most glorious task, sure, but as the Trademaster she does have access to a satisfying level of funds and more importantly, this will pave the way for a great future. She does seem very enthusiastic about this, although maybe a bit eccentric.
If the party talks to farmers, they’ll learn that no farmers blame the ankhegs. In fact, there is one that says Ankhegs can help enrich soil (which, a nature-savy character can confirm.) However, Trademaster won’t budge, saying that regardless, Ankhegs are monsters and occasionally damage farmland or cause issues — which also is true. Worst case, she says, it will be population control. The adults are dead but the younger ones will grow. That also seems to make some sense?
Truth is, the Trademaster is a shapeshifted/illusion using drow, who’s been pushing the village against the elven druids from the very start. Her plan is to get rid of the ankhegs because even more below the surface and outside the area the town stays are a budding colony of giant ants who are staying clear because of Ankhegs. Without them, the ants will move in, eating at crops and whatever they find outside, creating more pressure she can use against the druids. But her ultimate reason is something, or someone, else: A giant, sentient spider she’s tasked to engorge, who is looking very much into a colony of ants to be closer. She plans to get the town and druids to fight each other while making the spider stronger. She’ll confront the leader of the circle and let the spider eat the elf as an offering to Lolth, then call in her clan to move in from the Underdark through the ant colony’s corridors and attack the town from inside, slaughtering everyone in Dark Mother’s name.
Will the party realize something’s off? Will they try to find out more?
DM can implant as many clues as they like. Maybe farmers hire another group to oppose the adventurers. In fact, maybe the party are from the town and partially witness the transformation of it. It may make the trademaster even more reliable as she puts on the façade of an ambitious, stern but good-intending merchant and leader. This could tie in to an underdark campaign/adventure as well.
At the end, the party might have a powerful enemy and an ally in the Trademaster and the Druids, depending on how they act.
Ankhegs are a bit low level, but it could be an interesting idea to maybe make them a bit more powerful and say they are a bigger type or older or enhanced by druids (who might have done it because Ankhegs can indeed help farmers). Or who knows — maybe there is a shadow druid in the circle, secretly acting against the village.
Either way, this adventure can be a fun, one-session side gig or be part of the world with varying consequences for short and long term.