From the Introduction:
True heroes cannot be stopped. Nothing will stop a true hero from reaching their goal. Maybe you can hurt them; but their fighting spirit, their indefatigable determination, makes them invulnerable. They do not give into fear or despair, their inner demons, or the whisperings of others. Their devotion to justice, freedom, protection of the weak carries them through any ordeal.
True heroes are dauntless.
The Dauntless sourcebook is a companion to the Invulnerable RPG, like a sidekick. A smaller, scrappier brand of adventuresome goodness, to enhance and banter with the bigger, beefier core rulebook. Sure, the core rulebook is all you need, but sometimes, with another hero, you've got a one-two punch that finishes off the villain and saves the day.