Boyg is a word that Henrik Ibsen (Norwegian Playwright) used in his play Peer Gynt for the unknown fear that consumes our darkest thougts, manifesting itself as a palpable force in the life of Peer Gynt, a nomadic adventurer. Lots of symbolism and social commentary disguised as allegory, etc... To defeat the Boyg, Peer must (surprise!) meet the fear head on (instead of going around the hill), but the Boyg is tricky. It uses Peer against himself, getting him to beat himself up, gnaw at his own arms, all the while thinking it is the Boyg's doing. Peer is saved by the [church] bells (not for the first time in the story):

BIRD-CRIES
Boyg, there he's fallen! Seize him! Seize him!
THE BOYG [shrinks up to nothing, and says in a gasp:]
He was too strong. There were women behind him.

Peer goes on to have many more adventures before the end of the play. Boyg is also a Norwegian word that means curve or bend.
Mom's vision of the boyg is dark with red eyes and rumblings and foggy swirls that consumes people in the night. I see it as a dark pond filled with fish at night. Everyone has their own vision of the ultimate fear. This is unlike hell, where most people's vision (the flames and torment) come from Dante's Inferno, level 7, for the violent, assassins, war-monger, and those who have committed crimes against god and nature.
So really, if hell becomes personal, then is our personal ultimate fear really our personal hell?