As you shine your torch through the broken wall you take a step back. Holy shit, a Maw of Despair! The room is filled by this huge vile unholy creature. Its stony body holds giant teeth that grab and imprison. Unable to move much it mostly prays on creatures falling in its maw. But when hungry is known to lash out violently from its position. It looks like the maw mauled through the floors of the tavern. From its maw you can see the hands of taverners trying to get out. But the maw is slippery and nearly impossible to get out. Others gave up and moan as they slowly accept their fate while the stomach acids slowly eats away at their bodies.
How did it get here? They never move from the place they settle in and only grow bigger the more they devour. At its base you see traces of a summoning circle. Who would summon a hungry maw of despair under a tavern? Behind the monster you see a hole in the wall that leads into the sewer. The necromancer must have escaped into the sewer after activating the circle.