This video shows you very detailed what needs to be done to find the ancient demon Hybris and how to win the final fight.
You need to find 3 different scrolls in act 3. Every scroll will give you a side quest. Travel to the hidden lair, marked on your map and kill all enemies. After you finished all 3 quests for all 3 scrolls you can enter the hidden lair in Darktown. Inside this lair is Hybris. It's good to have a tank, a healer and 2 damage dealers with you. Make use of your potions if you are forced to do so.