The Infected Realm was discovered by one of the wizards that helped make the Fountain of Heroes.

Around 200 years ago the Wizard opened a portal to newly found dimension in an alternate plane to the normal world.
He originally sought to control this Dimension and its powers to use for good things. Although it wasn't long before the power overtook him.

This event started the Corruption War and destroyed most of Old Oivarch.
Corruption a the Source of Evil 

The Infected Realm, also known as the Nether, has multiple entrances but their locations are hidden and lost to time.
Some say the corruption is preparing a new wave of attacks and that there's a new portal being created.

It is rumored that there is a small civilization of Infected Realm natives still fighting the Corruption inside the Infected Realm.

Some of these natives, also known as Pig-Men, have managed to escape the Infected Realm during the Battle of Old Oivarch but they are allergic to sunlight so most of them moved to the dwarven mountains.