I've always heard politicians and journalist call them "Black blocs", but I've found that that's just the name of the tactic that they use (which is the same used by antifa - dressing in black, using ski masks and sunglasses to conceal their identity, etc) and not the name of a specific group.
They've been around for as far as I can remember, certainly since the very early 2000s. They mostly engage in property damage and shit like that, and disturb big left wing protests. I've heard of them doing pretty wild shit at Yellow Vests protests in France, G8/20 protests (off the top of my head - Genoa and Hamburg), and the 2011 London riots. Sometimes things escalated and people died.
They're certainly organised on some level, but I don't know how much.
I don't know if it's different in the rest of EU, but most of "antifascism" here is just throwing concerts with serbian folk bands and smoking weed while you say "right wing PM bad". N-not that I was a bit of a commie in high school.