Why?

Cleansing Area -> Fractured Orbs
30/40 Atlas Skill Tree Points

How To Find?

Within the mist of wraeclast, the fog of war however you wanna call it, check the top of the buildings that breach the mist/fog. If they have red tendrils that's where you want to head out. This red tendrils are the influence of the corruption, therefore a nexus is nearby.