"This ancient
and mysterious charm conjures a magical guardian, a projection of all
your most positive feelings. The Patronus Charm is difficult, and many
witches and wizards are unable to produce a full, corporeal Patronus, a
guardian which generally takes the shape of the animal with whom they
share the deepest affinity. You may suspect, but you will never truly
know what form your Patronus will take until you succeed in conjuring
it."

—Miranda Goshawk

The Patronus Charm is the most famous and one of the most powerful defensive charms known to wizardkind. It's an immensely complicated, very difficult spell
that evokes a partially-tangible positive energy force known as a
Patronus (pl. Patronuses) or spirit guardian. The charm was primarily
designed for defense against Dementors and Lethifolds, against which
there is no other protection. Albus Dumbledore,
however, invented a means of using one's Patronus as a messenger to
other witches and wizards, and this is the method of communication that
the Order of the Phoenix used in time of the Dark Days and in Voldemort's Return.

Dementor

A Dementor is a non-being and Dark
creature, considered one of the foulest to inhabit the world. Dementors
feed off human happiness, and thus cause depression and despair to
anyone near them. They can also consume a person's soul, leaving their victims in a permanent vegetative state, and thus are often referred to as "soul-sucking fiends" and are known to leave a person as an 'empty-shell'.

The Ministry of Magic
employed Dementors as the guards of Azkaban prison, until mid-1996,
when Lord Voldemort was sighted in the Ministry, and their defection to
his side was realized. The Dementors supposedly led the Death Eaters and
Voldemort into the Ministry of Magic.
After the end of the Second Wizarding War in 1998, the Ministry was
reformed, and Minister for Magic, Kingsley Shacklebolt, ensured that
they were not used by the government again.

There are certain defences one can use against Dementors, specifically the Patronus Charm.
Dementors hold no true loyalty, except to whomever can provide them
with the most people to feed off. They cannot be destroyed, though their
numbers can be limited if the conditions in which they multiply are
reduced, implying that they do die off eventually.

Lethifolds

ts appearance resembles that of a black cloak roughly half an inch
thick, although it gets thicker if the Lethifold has recently digested a
victim. It glides along the ground and other surfaces in an unknown
form of locomotion, in search of its prey, humans. It attacks its prey
at night, when the target is asleep, and suffocates and digests it in
its bed.

The only form of protection against a Lethifold is a Patronus, similarly to a Dementor. Other spells such as the Stunning Spell will not work.

The Lethifold is a very rare species, and can only be found in the tropics. The first recorded survivor of a Lethifold attack was a wizard named Flavius Belby. Belby was on holiday in Papua New Guinea, and he managed to escape because he was not fully asleep at the time. He repelled the Lethifold with a Patronus Charm, something other victims have been unable to do because they are either asleep or Muggles (or both). It is impossible to determine the number of Lethifold victims, as they leave no clues to their presence.