Last June, Islamic State militants forced 100,000 Christians to flee the Iraqi city of Mosul in a single night.

In an interview with the Daily Express, Iraqi priest Father Douglas Bazi shared details of how the city's population was given a 24-hour ultimatum to convert to ISIS' radical Islam or face execution.

He said 100,000 Christians chose to flee, penniless, leaving their homes and livelihoods behind. Those who stayed behind had to pay a Jizya – an Islamist tax for non-believers – of more than a year's salary for an average Iraqi. Those who could not pay were beheaded.

"Before 2003 we were two million Christians in Iraq. Now we are maybe 180,000," Bazi said.

"At least 1,800 Christians have been killed since 2003 in Iraq. That's why the people are afraid.

"Overnight 100,000 people escaped from Mosul and escaped from ISIS. When they arrived and took over Mosul they told the people we have three conditions according to Sharia.

"Number one was convert. The second was to pay Jizya and they asked for each person to pay 4,000 to 8,000 US dollars. Third, they said you have to leave or you will be beheaded."

He described the situation for those who decided to stay in Mosul, sharing one case in which a jihadi fighter forced a dentist to accept a female sex slave as payment.

"The dentist helped him and the man told him he would come back to pay him. The dentist said he didn't have to, but the man insisted he did because it was Sharia.

"After a couple of days he came back and he brought him a Yazidi girl and said 'this is your reward'. If the dentist refused her, he was going to shoot him, so he took the girl and managed to reach her family.

"When he told the family 'I have good news for you, your daughter is here how can I bring her to you', he was shocked because the family told him she was not a virgin anymore, so keep her, we don't want her.

"This girl, how can she live anymore when she is not wanted by anyone? If you look at the minority in Iraq, this girl is symbolic of our life."

Bazi also shared why he thought ISIS was the most dangerous branch of radical Islam that is yet to emerge. "If al-Qaeda saw an American soldier and he was distributing candy to the kids, the al-Qaeda sniper is forbidden to shoot him because the kids might get hurt," he said.

"[Former al-Qaeda leader] Musab al-Zarqawi was the second generation. He said don't worry, shoot him and if you're going to kill the kids don't worry about them because those kids are going to be angels in heaven.

"The third generation is ISIS. They said kill the Muslims, not just the infidels. They started with killing the Muslims that aren't according to Sharia, so imagine how they look at us."

Bazi said many Christians were too afraid to speak out of the atrocities they had experienced for fear that ISIS will attack their family and friends who are still in Mosul.

"There is no life left behind for us now", he said, describing the way in which the city is now run by two rival "gangs" of ISIS fighters.

Bazi was captured and tortured by al-Qaeda in 2006 while walking home from a mass in Iraq. Since his release, he has founded centres for displace Christians in a safe zone in northern Iraq.