To do so, run past the fighting enemies until you reach a statue of Vaermina.
Veren Duleri will ask you, a priest named Casimir, to release the Miasma on the orc invaders. Consume the item in your inventory to start a strange flashback sequence where you take control of a Vaermina priest. Upon bringing the potion to Erandur, he will ask you to drink it. It will be residing on a shelf on the laboratory's bottom floor beside a whole host of alchemy ingredients. Vaermina's Torpor rests in the laboratory next to the library you found The Dreamstride book.
In-game, it has the appearance of a Potion of Ultimate Healing with a darker shade of red. Erandur will describe the potion as a small flask.
