Complex sentence: An unpublished 1940 story titled "The K-Metal from Krypton", which Superman creator Jerry Siegel himself wrote, featured a prototype of Kryptonite, a mineral from the planet Krypton that drained Superman of his strength while giving humans superhuman powers.
Split into simpler sentences: An unpublished 1940 story titled "The K-Metal from Krypton", written by Superman creator Jerry Siegel, featured a prototype of Kryptonite. It was a mineral from the planet Krypton that drained Superman of his strength while giving humans superhuman powers.