The cat goes in to the dungeon with him and is gifted with sentience immediately, becoming the protagonist's sassy partner. It's cheesey fun.
As others have pointed out, the book has a lot of over the top violence and death, so you might want to stay away. On the other hand, though, the author is far more into gore and comic book ultra violence than cruelty. Many people find Princess Donut -- the cat annoying -- but by book five no great harm has befallen her or any of the other protagonists.