His crime was killing and impersonating all the older dopplegangers in his tribe and killed and impersonated the leader of the tribe.
His curse is that although he was granted the ability to assume any form he must do so every few seconds.
He also has a strange lust for feeling emotions of terror but he also has a resurrection touch so anyone he murdered would come back to life and he would only experience the emotions a for a brief second.
The Fang of Nosferatu was given to him as it could extract those feelings but he became so agitated when using it that he shifted so much that he couldn't use it. He found out that if gloves made of doppleganger skin were used by the murderer the emotions would stay in the knife so he could experience them after someone was murdered with the knife by one of his agents.
As a side perk, it granted 13 years of invulnerability to the first person who touched it after 6 murders on 6 consecutive nights were commited. On the thirteenth cycle, the doppleganger who commited it betrayed him with the intention of getting the invulnerability himself. It is up to the characters to find out who this is and to stop him from commiting the last one.