Essentia Obscura

Essentia Obscura is a liquid that inflicts Insanity on players who try to swim in it. It exists as puddles in Elder biomes, and some monsters pour this liquid when they are defeated.

It is also an average fertilizer for Trays. However, using it might be suboptimal, because its Fertilizer bonuses are exactly the same as those of Organic Soup, which is much easier to produce.