Upon killing them, their bodies dissolved into organic masses of flesh which flowed down a hallway and merged. A monster formed from the merged remains and grew as it ate the remaining organic matter.
This creature was under the control of the Mind Flayer. The creature then began attacking the pair, chasing after them through the building.
While pursuing Nancy, she closed a door on it; however, the creature simply melted and crept underneath the door, re-solidifying on the other side. It attacked Nancy while Jonathan tried to break into the room.
Eleven vs Monster
At the last second, Eleven arrived, breaking down the door with her powers before fighting the beast, beating it to a pulp. After being defenestrated, the creature slinked off into the sewers, where it would be re-absorbed into the Mind Flayer’s proxy.