Defeat Pride.

Story-related; unmissable.

You will find this particular Sin at serpent hole Haven, South End.

Combat tips: Before you can unload power Hollow strikes or Havoc mode, you must take her shield down first. When she reaches half her health, she'll bring into the fight a floating skull that continuously fires a steady beam of energy. Focus on Pride with 'LT'. Once her shield is down, attack with 'X'/'Y' combos and if you have any handy, use fortification and strength shards. Also, I found very useful the Storm Hollow 'LB' + 'RB' attack. It'll keep her stunned while you plan your next attack.