Actually no. That was done by Valar after an elf refused to give up gems that he only could make that radiant thanks to the light of the trees (after the trees were destroyed by Shelob's predecessor Ungoliant, who was in pact with Morgoth).
I have not played the Shadows-games (well I started Shadow of Mirror but it absolutely didn't hook me so I left it quickly), so I'm not well-versed in those games' lore.
However, I looked it up and it seems the developers made Shelob a Maia like Sauron himself, and Sauron like too she can shapeshift in Shadow.
On how Samwise was able to defeat her: he used the phial of Galadriel that contained the light of Eärendil's Star (which is a actually the light of a Silmaril on his forehead) and in Tolkien's lore light is often detrimental to evil creatures (especially such a powerful and pure light). The light from the phial caused pain to Shelob and Sam wounded her with Sting, an old and powerful elven blade. However, in the end he couldn't actually overcome her and she defeated herself by trying to crush Sam who simply held Sting upright between himself and her and she lowered herself onto the sword. It is also never told whether she actually died from that wound, she squibbled away and was never mentioned again.