Town in Shadow

"The town you venture towards seems almost to good to be true. A mass of traveling caravans have all arrived and are setting up shop to sell goods from every corner on the region. A massive weapons expo distracts your melee fighters while magical trinkets of all kinds catch the arcane eye.

All seems well as you see a young girl call her small dog into the house after a day of sight seeing. The most observant of your party stumbles on an exact twin of the dog later that night... Dead and torn open only a couple of hours fresh. It's then that they spot something truly unnerving... Every shadow, every darkness, has an oily and viscus look, and when spotted, it moves... Collecting into the house of the dog that was called home."

haha My description may have gotten a little heavy handed, but envision this with me: As the weird liquid oil gathers and spills over the little dog, causing it to grow as it wretches around the house. Finally pouring out onto the street, its horrific form takes shape into that of a viscus monstrosity. Bubbling into this massive figure that wants to consume everyone and everything.

This would be fun and could be scaled to any level, just using different/modified stats of Shambling Mounds, Oozes, or a homebrew creature to really throw high level players off!