The ground trembles and gives way beneath you. Buildings topple and collapse to either side as you tumble into a massive sinkhole. To your horror you see a group of Fungal Zombies clawing their way from underground. Their mushroom encrusted faces scanning for flesh.
The ground collapses into a giant sinkhole. Releasing 2d10+4 myconid zombies. Who attack the party.
After the first round read the following.
Before your eyes the walls and ground begin disappearing under a flood of green vines that begin repairing the damage and closing the holes. As you watch several large green bulbs start sprouting from the walls and pivot towards your group. They split open shooting waves of projectile thorns, hitting adventurer and zombie alike.
At the start of every round the entire party and all zombies make a DC 14 Dexterity saving throw. On a failure they take 4d8 piercing damage, on a success they take half.
The barrage stops once all zombies are dead. The party may choose to enter the underground and go into the sewers from their current location. Arriving at the closest sewer hex to their position.
After 1 hour the plants have completely sealed the sink-hole so it can no longer be used as a passage to the sewers.