Create a copy of the sword (trailsword) and give it the fade behavior and set it to like 0.1 seconds to fade out
while dragging
every 0.1 seconds -> create object trailswrod at sword.x sword.y
set angle of trailsword to sword.angle
now while you drag the sword around, it will create copies of the sword 0.1 seconds behind the position of it. resulting in a dynamic trail effect.