It was filmed last week by one of those fighters, the Briton Thomas Evans.
Intended for use on a website to boast of the jihadists’ latest success it instead documents the last violent moments of his life.
For two and a half years his group had been operating, apparently at will, along the Kenyan border with Somalia, al-Shabaab’s base.
Last year they visited hell on the town of Mpeketoni killing, say witnesses, seventy civilians.
Survivors told me that they can never forget the sole white man among the killers.
"He had a machete and a gun," recalls Kimothe Ngorgoe, whose son was among those butchered.
"I saw him slice the neck of one of our watchmen. I don’t understand why a British man would come here to kill us."’
There are many foreign fighters drawn to al-Shabaab’s cause; perhaps fifty from the United Kingdom.
Evans was unusual only in that he was a Christian convert to Islam who went down a now wearyingly familiar path of radicalisation.
The video he shot, which includes a sequence of his unit training in the Kenyan bush, is now being studied by intelligence services in Africa and Europe.
The rise of ISIS has in many ways overshadowed al-Shabaab’s campaign of terror.
There are easier wars for British jihadists to reach than that in the Horn of Africa.
But as Thomas Evans’ last video demonstrates it is still deadly.