Mother was the first Guardian, leader of Nika City and the one who faced the onset of the corruption. She set forth to find a way to eradicate this curse before it bled into Nika City and doomed her people as it was dooming all around.
She was a smart leader, relying on the skills and insights of all her greatest citizens. However, many did not survive the process of unearthing the truth of the corruption. Worse than death though, was that some befell the curse during their expeditions.
Eventually, Mother too was cursed. Her journey to purge the evil soon spanned lifetimes as it permeated the lands, eroding through her will and mind. She scoured every culture, every ruins, every faith in order to find a means to escape the curse on herself and her people.
When finally, one day, she seemingly snapped. She returned to Nika City and meticulously killed everyone, devouring everyone through a dark ritual.
This is the major strokes throughout the narrative, leading you to believe that Mother failed in her journey and became mad from her eternal life.
The truth however, is that she was unable to find any way to thwart the corruption. Instead, she learned of a dark ritual that allowed one to consume the souls of others and bind them within their own spirit. With no other options left, Mother consumed the souls of the Vorku, extinguishing her people but protecting them from the agony of the eternity she was forced to face.
This was an endless time ago, now only she remains, a twisted and distorted monster sealed away in the Inner Sanctum. All guardians after, the crone, and any Vorku guides that traverse the land are projections of those spirits bound within her. They are safe from the curse of immortality as they remain within her.
All guardians after are simply souls of the Nikans now granted a form that matches Mothers strength and power.