In Smash bros and many other games, it appears as though weapon trails are creating a plane along the path of the swing, How would this be done in something like unity? It also seems like there is some extra smoothing going on, since the individual edges are smooth. It also seems dynamic in many games (like ocarina of time).

For Unity, the component you'd be looking for is the trail renderer. It essentially builds a mesh behind the object it is following as the object moves. This mesh can have a material/textures applied to it to create fuzzier/smoother edges.