The Levin Sword appears in the Fire Emblem Series and allows the wielder to channel Thunder to attack a given target. Instructables user queenofpuns created a replica and used LEDs to match the look of the sword as seen in the game. You can see a closer look at the sword below. It took her about 28 hours to build. She crafted the sword using modeling clay, fiberglass cloth, fiberglass resin, Apoxie sculpt, plywood, and a handful of other materials. It was her first attempt at propmaking, and she was resourceful. She started by creating positive molds. She says:

It helps to print out a true scale template first (i just used an image from online, attached here, and scaled it myself. try to scale the sword so that it will fit with your height, mine was about 91″ long and I’m 5’6″)
acquire two pieces of insulation foam roughly the size of your molds
outline your template and bevel lines (copied from the template printout) onto one piece of the foam, then stack the two halves and use a hot wire to get the basic outline. since the other half is a mirror image, make sure to reverse your outlines and bevels lines as you prep the second half.
use a hot knife (in my case, a hot exact-o knife) to carve your bevels! make sure to align the pieces every once in a while as you carve to make sure they’ll be even.
you can choose to fill in any imperfections (aka places where your hot knife decided to do whatever it wanted) with modeling clay. remember that you will have to clean the clay out of your mold later, and it will be an annoyingly tedious process.
fiberglass resin will dissolve your foam mold, so seal it in epoxy before casting. they take about 12 hours to dry. do this on wax paper so you don’t ruin your work surface!

Read about the full build process at Instructables.