This knife was one of the "wall decorations" hung up in Euronymous's shop Helvete, it can be seen in the "Lords of Chaos" book as well as in the movie "Until the Light Takes Us". It was taken by one of Euronymous's friends immediately after the murder. I obtained it from him. Anyone who ever visited the shop will immediately recognize this. You can see a still from "Until the Light Takes Us" below the pic, the rather ugly, touristy looking knife is on the top right hand side. In a very, very bad quality.
Film still from "Until The Light Takes Us", showing the knife: