Nexa is the Lord of the Undines, master of all the water elementals and for many thousands of years, he ruled the world in the time before men, in the time of the Great Old Ones as one of the most powerful of the Primal Gods. Along side Cthulu and the Keeper of the Nameless Book, Nexa lived in a world without humans, moving from ocean to ocean, taking great long sleeps, always awakening in a new body of water.
Eons ago, Nexa once again entered a slumber and this time, found himself in waterways of New York, deep at the bottom of the cold and murky East River, which would have occurred shortly after the river was formed about 11,000 years ago when glaciers carved it out during the end of the Wisconsin glaciations. He rested peacefully there until approximately the summer of 1985 during which he was awoken by pipe layers, workers for the city.
That summer was marked by a powerful heat-wave in which temperatures soared above a hundred degrees, turning Manhattan into a virtual oven. Nexa, however, was less than thrilled at being awoken to a world that was much changed and in his rage, he unleashed scores of undines which possessed the city’s water supply, causing havoc and panic not to mention water shortages in the already dangerous heat.
The Ghostbusters were called in at the request of Mayor Clotz to investigate the disturbances and put a stop to the problem. Dr. Spengler quickly deduced that Nexa himself, not the Undines, was responsible for the attacks and the team tracked Nexa down to the East River. Nexa nearly defeated the Ghostbusters, swallowing them alive like a modern day Jonah's whale, but made the tactical error of spitting Peter Venkman out, much to the surprised parapsychologist’s relief and half-disguised irritation.
Peter, in a moment of technical euphoria, created a unique nuclear powered microwave emitter filter that amplified the wavelengths of the particle stream and used it to “negotiate” with Nexa, holding the gigantic being hostage with the threat of dehydration unless he released the other three Ghostbusters, which Nexa promptly did, agreeing that he would leave the world of men and seek out less populated waters in the depth of the world’s oceans.
Nexa himself was corporeal being capable of paranormal generation of Undines, which he housed inside his massive body. Standing the height of a small building, Nexa was titanic in both size and power. He was green and black, with a massive tail, giant talons and a thick rotund body. Twin horns rose from his head and his red-blue eyes pierced all who beheld him. While bipedal, Nexa spent most of his time at the bottom of the body of water he had drifted into.
Due to his status as an Old One, Nexa is most likely at least a Class 7, possibly higher, though this is never made clear, despite Egon referring directly to Tobin’s Spirit guide. With Nexa and Peter reaching an agreement and Nexa leaving the world of men for less in habited shores (and ostensibly to find according to Peter a “cute little Nexette”), this marks the only time in the entire Ghostbusters series in which a villain Primal God is able to reach a peaceful resolution with mankind.
Nexa is probably still out there today, most likely in the deep Atlantic, near the Mariana Trench, one of the deepest locations on Earth. No new reports have surfaced of Nexa so it is assumed he found his rest at last.