The polar ice caps contain enough mass when molten too flood the majority of the Earth's landmasses. This would quite probably result in the death of 80% of the world's human population. So Zera could rather effectively wipe out most of humanity without ever leaving his house, forcing the heroes to go find and stop him.

Alternatively, you could have him do all kinds of other evil stuff. Such as collecting magical artifacts (dragon balls/keyblades/halos/whatever) which will somehow grant him the power to end the world, or do something else really evil. Give it a cool Sea theme. Maybe he'll find whatever sunk Atlantis, and use it on the rest of the world.

Get creative. Find a way to involve other characters' personal story arcs if you can. Get the villains involved, either as rivals or allies of Aquilus. Make sure the heroes have enough power to have agency, and enough freedom to determine their own plan of approach.