Beneath the City Streets: A 4th Level Adventure for 5th Edition

The sludgemaw goblin tribe used to call the swamps just outside Valhenge their home. In the ruins of the old city, long since abandoned by the city folk, they built their stronghold. Besides the occasional raid on passing by caravans and defending their home from would be heroes looking to collect their heads for a bounty, they pretty much kept to themselves. 

One day though, one of the goblin scouts discovered an unused and forgotten entrance into the city's sewer system. Eager to explore, and perhaps secure a more formidable base, the leader of the tribe, Magruk the Filthy, an ogre and toughest among them, declared he would lead a party into the sewers to investigate the tunnels below. With his trusty basilisk, Groggy, by his side, along with a contingent of the fiercest warriors the tribe could muster, he headed into the sewers. They spent days underground, searching for the right place to call home. When they finally found it, however, they realized that the way back out was now blocked. 

A swamp hydra, disturbed by the commotion around the old sewer entrance, had followed them inside and now stood between them and the exit. Unable to face the threat, Magruk had his men seal the chamber the hydra was in and hole up in a corner of the sewers. When the hydra failed to leave, Magruk began sending his scouts to the surface in search of a another way out. When goblins began bursting out of the sewers, the townsfolk did not respond positively.

The town guard has since sealed all the entrances to the sewers save for one and have congregated around it to hold off any more goblin "invasions". With food and supplies running low, Magruk and his kin are becoming desperate. Unless someone clears the way for them, they'll have no choice but to charge the surface and fight their way out of the city, causing untold damage and costing countless lives.