Sludge

More or less the main antagonist of the campaign so far, though we as a group know next to nothing about it. We know that the sludge is sometimes called the "Quolvo" - it was first referred as this by the dragon Massasauga, and later the name was recognised by Yash'ailaran in the Shard Dimension.
In the insightful words of Lysander, the sludge is maybe intelligent, maybe not. It does seem though that they communicate as some kind of hive mind. They come in a few different forms, some use ranged attacks, some fight in melee.
The sludge seems to be weak to both extreme heat and extreme cold, making it fairly vulnerable to a spellcaster.