A website named daily beast reported a concrete sports stadium situated in Raqqa, Syria was used as an execution ground by the ISIS. Raqqa was recently freed by U.S forces deployed in Syria.
Civilians were brutally tortured by iron cables, and many were beheaded. Hence the stadium is also known as "The Black Stadium".
A 42-year-old native civilian, Xabur reported to the website "The Beast" that 80 percent of civilians who were kept in the prisons were beheaded brutally. As some of the reporters from the website moved to the stadium, they could find iron cables and plastic straps to tie and kill the prisoners. There were three tunnels, many jails, and a gym. The walls of the stadium were found with many different languages scribbled on it. Some of them were Russian, Turkish, Kurdish, Arabic and English.
According to Xabur, those civilians were prisoned were either the unbelievers of Islam or were against ISIS. Those people were hit by the tiers and even hanged from the roof.
https://www.thedailybeast.com/secrets-of-the-black-stadium-in-raqqa-inside-isis-house-of-horror