Nexus is a synthetic race created by Epsilon Project. First the were only observing the facilities but after Batarian rebel they been stationed on a lone planet on Tynor with a small observation of them for future Epsilon projects. They look like as geth in one way but they are a lot more different to them. Inside of them house a different system. Not neural network but they use Controled nexus system.
Need of creation: Edit
Mostly the Epsilon needed the nexus to protect their facilities. And they did for thirty years, but at the point when Batarians rebeled they were sent to Tynor for futher observation for possible future creating the organic races.