Five months ago I wrote an article for TeleSUR English following the story (more of a meme, really) about, and this should pronounced as a single phrase, "Afghan Shia Militias in Syria". I compared it to the older story about Gaddafi's "African Mercenaries", used to good propaganda effect in that war. I spent some time trying to get to the bottom it and find what sources writers were basing their stories on when they wrote about the "Afghan Shia Militias in Syria". What I found was thin indeed: anonymous Syrian opposition fighters who talked about facing off with these (fast running, death-defying) Afghans on the battlefield; pseudonymous Afghan fighters who told journalists unverifiable stories; and finally poorly-sourced statements by anonymous Iranian officials. Based on these shoddy sources, journalists were building up to some outrageous conclusions: that the Afghans were an "inexhaustible reservoir of the desperate", that they "run faster" than the Syrians they were fighting, and that they had the miraculous ability to "keep shooting even when surrounded."
There was an Afghan community in Syria at the start of the war; some of these Afghans did join the civil war on the government side. As for Afghan fighters from Iran, the most promising reports to continue following the story were on the Iranian side. There are millions of Afghan refugees in Iran; many of them (perhaps most) are Shia, from the Hazara ethnic group. Some of the young men from this group have fought with Iran's military in their own unit (the "Fatimeyoun") in Syria. Since my story came out in May, I have seen reports from Iranian news agencies about such fighters - specifically about their bodies being returned to Iran for burial.