Once they form a keymind(by consuming an entire planets ecosystem) they start -blam!- everything

Star roads aren't flood, they're precursor. And basically they are these giant road-like constructs that used to connect star to each other. And being made by precursors, they are indestructible unless you hit them with a Halo pulse. They were utilized in combat occasionally by using them to cut/smash through planets like butter.