Changeling nets are illegal technology that allow its wearer to appear as someone else entirely.
They take up a massive amount of power to use and prolonged exposure to its energy field can be fatal. As well as using a lot of energy, they also put out a lot of it as well which allowed Sinclair and Garibaldi to locate the assassin and hunt him down.
In 2257, Del Varner smuggled a changeling net onto Babylon 5 to sell to a Minbari Assassin. Varner was supposed to meet the transport before it arrived at Babylon 5, but they were unable to make the rendezvous. When the Minbari Assassin arrived at Babylon 5, he killed Varner and assumed his identity. A Babylon 5 technician, Eric, recognized the assassin-as-Varner and was killed. 
2. The Geometry of Shadows (Mentioned only)