"The thing attempted to smother me, sliding inexorably up my face, over my mouth and nostrils, but still I struggled, feeling it wrapping its coldness about me all the while. Unable to cry for assistance, I groped for my wand. ...Pointing my wand away from myself into the deadly folds of the creature, summoning the memory of the day I had been voted President of the local Gobstones Club, I performed the Patronus Charm. ... Almost at once I felt fresh air upon my face."
-- Flavius Belby on defeating the Lethifold (FB)
The Lethifold is a dangerous beast which resembles a half-inch thick black cloak which moves along the ground at night, hunting its prey. It is also known as the Living Shroud (FB).
It generally attacks sleeping humans, smothers them, then digests them, all in their bed, leaving no trace at all. The only known defense against a lethifold is a Patronus Charm. Thankfully, this frightening creature is rare, only found in the tropics (FB).
L. lethum, variation of letum, death (NSOED) + "fold," suggestive of cloth
The lethifold is referred to as an "Alethiafold" in J K Rowling's interview with Raincoast Books (March 2001):
"Then there's Alethiafold, which is the thing I would least like to be attacked by, which I think is quite a sinister creature. It slides under doors at night and suffocates its prey. So personally that would be my worst one." (RC)
The word alethia or alethic is derived from Greek alētheia (meaning "truth"), so Jo may have changed the name of the beast before publication to lethifold to bring the meaning back to something closer to "death-fold" (Oxford English Dictionary).
From the Web
Writing by J K Rowling on WizardingWorld.com (Pottermore): Celestina Warbeck
Harry Potter Wiki: Lethifold
Interview of J K Rowling by Raincoast Books: Accio Quote
WizardingWorld.com (Pottermore) features: