Attack on titan!

Currently I am watching attack on titan! Attack on titan is about how several hundred years ago humans were nearly exterminated by Titans. Titans are usually several stories tall and devour human beings just for the pleasure rather than as a food source. A small percentage of humanity survived by walling themselves in a city protected by extremely high walls. After about 100 years not a single titan got through the walls. A teenage boy Eren and his foster sister Mikasa witness something horrific as the city walls are destroyed by a Colossal Titan that appeared out of thin air, the two kids watch in horror as their mother is eaten alive. Eren then vows that he will murder every single Titan and take revenge for all of mankind.