Before working for Nintendo he was learning magic from some random people. When he learned how to pronounce spells he started reviving good people's corpses. He failed and caused a zombie apocalipse in Ancient Japan which would be over soon, when Giant Enemy Crabs invade it.
He joined Nintendo later and started developing shames about some guy called Mario. Then he used magic to force people around the world to buy the shames. He became RICHER later.After developing his magic powers he united with Napoleon Bonaparte to develope a device to revive people succesfully. After working together for some centuries they created the Revival Machine. Napoleon took the v1.0 machine. Shiggy added some changes and took the v1.2 machine. Nobody got the v1.1 machine. It was never seen by anyone.
After reviving a bunch of famous people Shiggy became very popular. Many people wanted Shigeru Miyamoto to become a new godlike dude but he refused. Instead he started stealing stuffs from the gods and giving them to poor and ill people. No god ever tried to get stuffs back (except for Superman. He is evil).
By the way, Shigeru Miyamoto is also a swordsman. He uses his sword to complete his ultimate quest: killing Link. People don't really know why but this must be to make the UnWorld a better place.