Hollowgasts, also known as Hollows, are creatures that were spawned from the Experiment of 1908. They are the secondary antagonists of the Peculiar Children series and film adaptation: Miss Peregrine's Home for Peculiar Children (film).
They were formerly peculiars led by Caul Bentham onto the path of immortality. In a bid to eliminate aging without the need for loops, a group of rogue peculiars led by Jack 'Caul' Bentham entered an old, unused loop in the Siberian wastes to perform the Experiment of 1908.
However, the Experiment went wrong and the peculiars became Hollowgasts. Hollowgasts are terrifying creatures who seek to consume peculiars, but will still eat anything if peculiars are not available, namely Coerlfolc.
|“||They only become visible to the rest of us when they’re eating, Miss Peregrine had told me, which is to say, when it’s too late.||”|
–Miss Peregrine's Home for Peculiar Children
Hollows are invisible to all Coerlfolc and Syndrigasti, save special peculiars such as Jacob and Abraham Portman.
Hollowgasts crave and feed upon the souls of peculiars, but they will eat Coerlfolc souls as well. If they consume enough peculiar souls, they become wights. They are only seen as a shadow to all, which is why they also tend to hunt at night, as the darkness hides their shadow, but a select few peculiars, such as Jacob Portman and his grandfather, Abraham Portman, can see the Hollows. Hollows can be seen when they are eating, which is when it's too late to save the person. A Hollow is seen to have tentacles inside its mouth and is taller than an average human. A Hollowgast must consume peculiar souls. They can eat as many as 100 Coerlfolc souls, but they cannot become Wights by consuming only Coerlfolc souls.
When a Hollowgast consumes enough peculiar souls, they evolve into Wights.