As the gods thought, Hades wasn't powerful enough to defeat Decim, and the moment Decim stepped into the mortal world, there was chaos everywhere.
Floods and droughts began to take place, there were tsunamis and earthquakes around the world and the only way to stop this would be to defeat Decim.
The gods got a prophecy one day that told them that in order to defeat Decim, they had to let him get more power first, and that power is what will cause his downfall. So the gods waited and waited until...
Decim got greedy and started to steal more and more power, until one day, his body couldn't handle it and it broke down.
When the gods heard that the great and all powerful Decim had died, they rejoiced and made sure that nothing like that would ever happen again. They had been saved by prophecy.
With Decim gone, the world went back to normal and there was peace again. Monsters like Decim never appeared again and Decim was sent to the fields of punishment to suffer the consequences for as long as he lived.