A large demonic serpent with a human-like face and arms rampages on the streets of Fellport, indiscriminately killing with its long hooked claws. It makes its way down the Lakeside Ramp from Level 5 down to the harbor, killing members of the Watch as it mounts a defense. Nobody reported seeing it anywhere above Level 5, nor on the Cliffside half of the city.
If the party wants to fight, they face a Wastrilith named Xarisha that has been held captive for years. It moves and breathes in water as easily as air, and it can corrupt the water it passes through. At this point, it wants to escape the city and find a watery place to corrupt and set up a lair. It enjoys its newfound freedom, and existing outside of the demonic hierarchy will allow it to grow into a recurring villain. If banished, it will memorize the caster's face and come after that person in the future if possible.
It has no treasure, though Lucius Modesto at The Dipped Pen will probably pay handsomely for components carved off the Wastrilith. If the characters investigate where the Wastrilith came from, either through Detect Thoughts or Speak With Dead, they discover a magical blank spot between escaping the clay jar in a storeroom and wandering the street on Level 5. Further investigation will get the party in the vicinity of the Forgotten Flat.