Syrian prison authorities at the infamous Saydnaya Military Prison near Damascus called it "the party", a sick euphemism for an alleged crime against humanity.
WARNING: Some readers might find the details in this story distressing.
This was the name for the weekly hangings held at the building dubbed "the human slaughterhouse", according to Amnesty International.
Victims would be collected from their cells and transferred to the basement of a red building, where they'd be severely beaten for hours.
Then, in the middle of the night, they were blindfolded and taken to a white building.
There, they were told they'd been sentenced to death, and within minutes, they were hanged alongside up to 50 other prisoners.
The rights group estimates up to 13,000 people died like this over four years to 2015, in a process authorised by "officials at the highest levels of government".
The Syrian Observatory for Human Rights puts the figure closer to 30,000.
So many were killed that satellite imagery showed what's believed to be a new crematorium built in 2017 to dispose of bodies.
Former inmates also described a series of "salt rooms" — makeshift morgues to preserve bodies in lieu of refrigeration.
On top of the death toll, many thousands more are believed to have been tortured, starved, and raped with "the apparent goal to humiliate, degrade, dehumanise and to destroy any sense of dignity or hope".
Prisons like this were central to ousted President Bashar al-Assad's ability to crush a civilian uprising, and they're currently the centre of a desperate search by families to find their missing loved ones.
Now, the world is beginning to hear from the survivors and demand justice for those who went in and never came out.
The frantic search for survivors
As Syrian rebels led by Hayat Tahrir al-Sham captured Damascus, they also threw open the doors of the Assad regime's notorious prisons.
The former dictator set up an industrial-scale system of arbitrary arrests and detention, where dissent was crushed in prisons that became synonymous with torture.
At the weekend, as his regime was overthrown, celebrations were followed by desperation, as families raced to the prisons where an estimated 130,000 people had disappeared during the 13-year civil war.
Relatives frantically searched through stacks of printed prison records, crowded into tiny windowless cells looking for familiar faces, and broke down walls in the vast underground complex of Saydnaya Prison.
They were searching for relatives that went "behind the sun," the Syrian term for being taken by the country's repressive security services.
And from the darkness, prisoners walked free; dazed, terrified, frail and unable to comprehend that Assad was actually gone.
Dr Mahmoud Mustafa from the Independent Doctors Association went into the prison and was shocked by what he saw.
"This prison has terrified people. On top of a mountain, so huge. We saw on people's faces how terrorised everyone was," he said.
"We saw the state of the cells and how they tortured people. I am still traumatised by all I saw. I don't have words to describe how I feel."
He's now treating freed prisoners at a mosque nearby as many are gravely ill and don't know how to find their relatives.
"Most of detainees have various medical problems," he said.
"They have developed chronic diseases due to bad medical care in the cells, it was scary. No human being should live in these conditions."
Ammar Asalmo from the Syrian civil defence group, the White Helmets, agreed.
"It wasn't made for a human because you can see dozens of people in such a tiny place," he told the ABC.
"There was no hygiene, it's miserable."
He said it was a time of both celebration and grief for so many Syrians.
"All the families I know were crying because they celebrate the victory, but also, they're missing their people," he said.
"Because [they have gone] maybe 10 years without knowing anything about them. Even my wife, two of her uncles are considered missing. They were taken by the regime."
The group is also investigating reports from survivors of a network of hidden underground cells at Saydnaya.
Mr Asalmo told the ABC they're offering a reward for any information.
"We are planning to ask people if they know of any secret prisons," he said.
"There's a reward for anyone to tell about hidden or secret prisons, because the number released from the prison is so small.
"It's less than 10 per cent of the number of missing persons."
However, the Association of Detainees and Missing Persons of Saydnaya Prison (ADMSP) refuted claims of detainees trapped in secret underground cells at the prison.
"ADMSP appeals to the families of the detainees and the forcibly disappeared not to go to the prison and gather inside and outside it, as this hinders the task of the teams working to find any evidence that can help uncover the fate of all the missing detainees," the group said in a statement.
"The liberation of Saydnaya Prison is not just a military victory, but a victory for all humanity. It is a symbol of triumph over the will to live over death, and the will of freedom over tyranny.
"It is a major step towards achieving freedom and dignity for all Syrians."
'Without justice, there is no stability in Syria'
Ammar Asalmo said while Bashar al-Assad and his family had fled for a new life in Russia, many of the perpetrators remain in Syria.
"Those people who arrest those [prisoners] and killed them, they are still living with us," he said.
"We need right now, justice, justice and accountability for this. Without justice, there is no stability in Syria."
He has urged the international community to support investigations into the brutal detention regime and the horrors that happened in prisons like Saydnaya.
"Now we need to help Syria establish our war crime court," he said.
"Right now in Syria, the big dictator has gone, we know that, but still, we have some concerns.
"We need the international community to help Syria to establish … this mechanism for justice and accountability so people can forgive.
"We believe in peace, but before peace, we need Russia to commit to helping us establish our mechanism for justice and accountability."
