You want a ribbon trail.
Decide on a number of frames of duration -- say, 30.
Then create a triangle list that draws a "ribbon" of that many quads -- this could be a triangle strip or list, with 60 triangles. This gives you 31 pairs of "top/bottom" vertices for the ribbon.
Then, each frame, you shift the position of each pair of vertices "over" one -- vertex 0 becomes vertex 2, vertex 1 becomes vertex 3, etc. Then find the top and the hilt of the sword, and set the top/bottom of the front of the ribbon -- vertex 0 and 1 -- to those positions.
This will give you a ribbon that covers the last 30 frames of animation of the sword. Texture some glowy texture, typically that is brighter to the top-left and duller to the bottom-right, across this ribbon, in blended/pre-multiplied mode, and you're done!