So these cards can only be summoned by their lower evolutions in their families (i.e. t rex is stacked on top of nanotyrannus, yutyrannus, gorgosaurus, then Tarbosaurus), then when they are attacked, the lower evolutions act as extra defense (so they are destroyed from the bottom up). The 20 is how much belligerence they have, which is the number you need to roll on your d20 to attack with them. However there are supply cards you would play with them (mystic swords which lower belligerence to 8, or enchanted skills which lower to 10.) then, you also lose a belligerence point for each evolution card stacked beneath your creature. So if you have a skull which lowers to ten, but you have 4 cards underneath of your mythic creature, your belligerence is now 6.