As you begin your decent down into Crawl you'll notice that your Monsters aren't doing too much damage to The Hero and thus you'll need to Evolve your Monsters. As the Hero gains levels, players who are using The Ghost will gain Vitae which will give you a chance to upgrade those simple Monsters for potential higher hitting more damage dealing Monsters, and with more tricks up their sleeves. As The Hero goes down the stairs, to the next level you'll have a chance to use your Vitae to level up your monsters.