After being held prisoner by the Islamic State of Iraq and Syria (ISIS) for nearly three years, Heiza Shankal is finally free -- and she has returned to her hometown to fight the extremists and "take revenge."

In the summer of 2014, as ISIS swept across a vast swathe of Iraq and Syria, a long-persecuted religious minority group to which Heiza belongs, the Yazidis, fled the militants' advance.

Many were killed or abducted by the militants near the Syrian-Iraqi border, but approximately 50,000 became trapped in the rugged Sinjar mountains.

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"When the massacre took place in Shankal (the Kurdish name for Sinjar) and ISIS kidnapped children and women, I was one of those who was taken away," Heiza says.

Yazidi fighter Heiza Shankal is shown cleaning her weapon in this screenshot from a video recorded on July 7, 2017.

The international community mobilized to come to the stranded Yazidis' rescue. Food and water were airdropped onto the mountains, and evacuations eventually took place.

But a different horror awaited the women and girls who were kidnapped by ISIS. According to reports, ISIS set up markets in several areas where non-Muslim women, including Yazidis, were bought and sold as sexual slaves.

"I was sold and bought ... in Raqqa, and I was finally liberated," Heiza says. When she finally regained her freedom, "I arrived to the hands of the comrades and they brought me to Shankal."

Back in her hometown, Heiza was ready to fight the group that had put her through so much pain, so she joined a women's resistance unit to battle ISIS.

"I was surprised to see a military force for protecting Shankal, so I decided to join the unit and take revenge," she says.