Earlier this month, the group said it had killed four aid workers it had abducted in north-eastern Nigeria.
Last year, Iswap killed two midwives it had previously taken hostage.
video https://www.bbc.co.uk/news/world-africa-50924266 (design)
It is also active in neighbouring countries, including Burkina Faso, Cameroon, Chad, Niger and Mali.
Dozens of hostages still being held
Ishaq Khalid, BBC News, Abuja
The video once again highlights the brutal tactics of Iswap, while the timing of the release - over the Christmas period - is also designed to get maximum attention.
This video may be intended to increase tension between Christians and Muslims in Nigeria, as well as put more pressure on the government to respond to their demands, says security analyst Kabiru Adamu.
Earlier this month, the group released a video of the captives, appealing to the Nigerian authorities and the Christian Association of Nigeria (CAN) to intervene.
Iswap has used hostage-taking as a bargaining tool - either for ransom or in exchange for their arrested members, although the authorities have never confirmed carrying out a prisoner swap.
The militant group has previously killed a number of hostages, including members of the security forces and aid workers but this is the largest group to be killed at one time.
It is not clear how many captives Iswap is currently holding in Nigeria, but there are believed to be dozens - mainly security forces, aid workers and those perceived to be associated with government institutions.
source / BBC