The Lethifold is a mercifully rare creature found solely in tropical climates. It resembles a black cloak perhaps half an inch thick (thicker if it has recently killed and digested a victim) which glides along the ground at night. The earliest account we have of a Lethifold was written by the wizard Flavius Belby, who was fortunate enough to survive a Lethifold attack in 1782 while holidaying in Papua New Guinea.

Near one o’clock in the morning, as I began at last to feel drowsy, I heard a soft rustling close by. Believing it to be nothing more than the
leaves of the tree outside, I turned over in bed, with my back to the window, and caught sight of what appeared to be a shapeless black shadow
sliding underneath my bedroom door. I lay motionless, trying sleepily to divine what was causing such a shadow in a room lit only by moonlight.
Undoubtedly my stillness led the Lethifold to believe that its potential victim was sleeping.
To my horror, the shadow began to creep up the bed, and I felt its slight weight upon me. It resembles nothing so much as a rippling black cape,
the edges fluttering slightly as it slithered up the bed towards me. Paralysed with fear, I felt its clammy touch upon my chin before I sat bolt
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. Now dizzy as the thing sealed itself about my face, incapable
of drawing breath, I concentrated with all my might upon the Stupefying Charm and then – as that failed to subdue the creature, though blasting a
hole in my bedroom door – upon the Impediment Hex, which likewise availed me naught. Still struggling madly, I rolled sideways and fell heavily to
the floor, now entirely wrapped in the Lethifold.
I knew that I was about to lose consciousness completely as I suffocated. Desperately, I mustered up my last reserve of energy. 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. I looked up to see that deathly shadow being thrown into the air upon the horns of my Patronus. It
flew across the room and slithered swiftly out of sight.

As Belby so dramatically reveals, the Patronus is the only spell known to repel the Lethifold. Since it generally attacks the sleeping, though, its victims rarely have a chance to use any magic against it. Once its prey has been successfully suffocated, the Lethifold digests its food there and then in their bed. It then exits the house slightly thicker and fatter than before, leaving no trace of itself or its victim behind.