Dark Realm

The Dark Realm was first introduced in A Link To The Past. In the Dark World, you are changed into a form that represents your true nature. Link was turned into a bunny in ALTTP, and Ganon was turned into a giant Moblin. When Ganon was banished to the Dark Realm and sealed there by the sages, he never left. He tried, of course, but he never fully escaped. Thanks to Link, he was either trapped there, dead, or stuck in between, with his power remaining in the Dark Realm.

Prevention
The only way to keep your shape in the Dark Realm is to recieve an item called the Moon Pearl, not to be confused with the Moon's Tear from Majora's Mask.