Dementor

A Dementor is a dark creature that resembles a hooded, skeletal figure.

It has scabby, grey skin, slimy, rotten hands, and a mouth full of sharp teeth.

It does not have eyes, but it can sense the emotions of its victims.

It feeds on human happiness, leaving behind only despair and misery.

It can also perform a terrible act called the Dementor's Kiss, in which it sucks out the soul of a person, leaving them as an empty shell.

Dementors are usually found in places of darkness and decay, such as the wizard prison Azkaban, where they used to guard the inmates.

They can also create a cold, foggy atmosphere around them, and force people to relive their worst memories.

The only way to defend against a Dementor is to cast a Patronus Charm, which produces a silvery animal that repels the Dementor.

Dementors are considered to be one of the foulest creatures in the wizarding world, and they have no loyalty or morality.

They joined Lord Voldemort during the Second Wizarding War, and attacked many innocent people.

After the war, they were banned from working for the Ministry of Magic, and their current whereabouts are unknown.

Dementors joined Voldemort because they were attracted to his power and his promise of more victims to feed on.
Dementors are creatures that feed on human happiness and soul, and they enjoy causing despair and suffering to others.
They were loyal to the Ministry of Magic only because they were given the prisoners of Azkaban as their food source, but they were not truly loyal to anyone.

When Voldemort returned to power, he offered the Dementors a chance to join him and his Death Eaters, and to have access to more people to torment and consume.
The Dementors agreed, and they left Azkaban to serve Voldemort.
They also bred more Dementors, creating a chill mist that spread across the country.

The Dementors were also drawn to Voldemort because he was a powerful Dark wizard who could perform the Dementor's Kiss, which was the ultimate way of consuming a person's soul.
Voldemort used this ability to punish his enemies and to create more Horcruxes, which were fragments of his soul hidden in objects.
The Dementors sensed the darkness and evil in Voldemort, and they followed him as their leader.

Therefore, the Dementors joined Voldemort because they shared his goals of spreading fear and misery, and they hoped to gain more power and prey by doing so.
They were not loyal to the Ministry or to the wizarding world, but only to themselves and their hunger.