Here is the full list of all 5 Pumpkin Eater achievements. It takes around 0-1 hour to unlock all of the achievements on Windows.
An organism dies, beginning the first stage of decomposition. Pallor, rigor, algor, and livor mortis will occur shortly.
The body begins to swell due to increasing methane gas pressure from anaerobic bacteria, forcing fluids to escape from orifices and wounds.
Bacteria in the body, starved for nutrients, will feed upon the body itself. Stomach acid leaks and begins to digest the body as well, not to mention the smells attracting hungry insects.
The body will release large amounts of organic material such as carbon and nitrogen into the environment. Butyric acid forms, essentially fermenting the body.
The final stage of decomposition. The body's remains will dry out and decomposition slows.