A misfire in cylinder 8 of your engine could be caused by a fouling spark plug, bad spark plug wire, bad ignition coil, fuel system problem or could be something internal to the engine on cylinder 8 such as a bad engine piston or engine piston ring.
If your vehicle has a separate ignition coil for each cylinder and spark plug then replace that ignition coil on cylinder 8 to see if the engine misfire goes away and if it does then that was the problem.
The ignition coil is the most likely and common culprit of an engine misfire that only happens on one cylinder.