The Last Ones Standing

Twenty thousand years ago gates appeared around the world and many kinds of monsters swarmed out of the gates, leaving humanity devastated and helpless. The people felt like humanity was over until a few amount of people were suddenly blessed with powers strong enough to defeat the monsters but most of the people died leaving only five chosen ones left, they were the strongest out of everyone and they established a key kingdom to maintain peace for their descendants and the people. Through time the descendants and common people started to get greedy and evil. Twenty thousand years later five descendants were unnaturally weak compared to everyone else but they made a promise to each other to never give up, but one day they overhead their father planning to kill them they ran away in hopes of getting away from their doom, but they were betrayed by their "benefactors" for money and were thrown into the forbidden zone that was swarmed with monsters ranging from low-level monsters to nearly undefeatable monsters, will they be able to survive and get revenge?