The Creeping Horak is an aggressively fast-growing Eternian fungus. When released onto a structure, it will rapidly spread to cover it completely, sucking out the oxygen in the process as all doors and windows are covered.
The Creeping Horak was used as a form of execution in a darker period of Eternia's history. Prisoners would be locked inside their homes as the Horak grew over them, allowing no escape. This is generally not done anymore as death by Creeping Horak is considered a horrible way to go, even for criminals.