Kreln was a mercenary taking work for various bidders when a job went wrong. He was tasked with breaking into an alchemist lab and stealing research data.
While doing the job he was caught and instead of being turned over or killed he was instead tested on by the alchemist. They were researching a serum meant to create humanoids capable of feats of super human strength and endurance.
While testing this they found it had the side effect of making the test subjects insane.
Kreln was an oddity in this where he was able to resist the insanity the drug caused. Eventually after resisting for months mostly being singled out for being able to resist the drug's effects he gave in.
What he remembers next is sometime later being in a place he doesn't recognize while not in control of himself. When he gave in to the drug a psychotic part of himself took over protecting Kreln from the drugs and stopping him from going insane but taking over control of his body because of this.
Over time he was able to work with this other version of himself and while they still have trouble effectively communicating with other people they have started to pick up adventuring contracts as a way to try to make a new life for themselves.