The Serum Supernus, otherwise known as The Ultimate Potion, is a concoction which is said to grant the unfathomable power to reshape the world to one's wish to whoever drinks it.
Due to its status as the ultimate achievement possibly obtainable by an alchemist, brewing it is the goal of Plague Knight and Mona in Plague of Shadows. However, Plague Knight also secretly desires it because he hopes that it can improve him just enough to being worthy of Mona's love.
To complete the potion Plague Knight must attain the Essences of The Enchantress, the other members of the Order of No Quarter and Shovel Knight. The true nature of the Essences remains largely unexplained, but they appears to be linked to a person strongest traits, such as Treasure Knight avarice, and their harvest does not eradicate said traits from the subjects but they instead only temporarily weaken. Essences are unstable in open air and they will eventually dissipate if not properly stored. To counteract this Mona developed a specialiazed Dynamo Decanter to contain them.
When Plague Knight successfully combined all essences, the Enchantress's Essence proved to be much more powerful than anticipated, causing an unpredicted reaction that transformed the Serum Supernus into a distorted version of Plague Knight persona, the Plague of Shadows and then the Corrupted Essence. Only with the defeat of both, the Ultimate Potion finally stabilizes.
The veracity of the Potion powers would however remain unproven as Plague Knight, having decided he no longer needs it, chose to use it's immense raw power to detonate it, levelling the Tower of Fate.