Ectoplasm

Ectoplasm is a translucent goo left behind when spirit matter no longer has a will giving it form. It first appears in Storm Front.

Demons and other entities from the Nevernever must gather energy to themselves to take form in the mortal world; when they are destroyed or cut off from the source of their power, that matter dissolves into ectoplasm.^[1]
Dust particles drifting through a portal between the Nevernever and the mortal realm will turn into ectoplasm.^[2]
Magical constructs will dissolve into ectoplasm when destroyed or cut off from the conjurer's will. Mouse destroys one such construct, a giant snake created by Quintus Cassius, leaving "gallons" of ectoplasm behind.^[3]

In the series[]

Storm Front[]

In Storm Front, Victor Sells sends Harry Dresden a scorpion talisman that comes to life at a giant size. Dresden is able to crush the scorpion with an elevator, squishing it and spraying ectoplasm on himself and Karrin Murphy.^[4] Despite being covered in it, the ectoplasm would vanish into thin air in a matter of minutes, returning to wherever it was magicked from in the first place.^[5]

Grave Peril[]

In Grave Peril, Harry Dresden and Michael Carpenter attempt to drive the ghost of Agatha Hagglethorn from a nursery in Cook County Hospital. Dresden shook some ghost dust onto her, causing her spirit flesh to tear away and fall to the floor as ectoplasm.^[6] Susan Rodriguez asks Dresden for details about the incident, but he brushes it off, indicating he only got "a couple bruises from slipping in ectoplasm." ^[7]

After a confrontation with The Leanansidhe and some Hellhounds in the Nevernever, Dresden and Carpenter make it through a rift back into the nursery. Both men were covered in some matter from the Nevernever, which quickly converted into ectoplasm.^[8]

In a discussion with Bob, Dresden talks through several possibilities for the identity of The Nightmare. Starting with the basics, he indicates ghosts can make a body out of ectoplasm and manifest in the real world.^[9]