The Guildmaster

TheGuildmaster is head of the Thieves' Guild of Bajapur. The position is currently held by Tozawa Nobusuke.
Nobody is aware that the Guildmaster has actually been a single individual for several hundred years, with his true name carefully buried. A former head of the Guild was an evil wizard researching immortality while using the Thieves' Guild to advance his aims. He pretended to be an arcane trickster to infiltrate the Guild, and soon achieved control.
With the resources of the Guild at his disposal the nameless wizard's research was vastly accelerated, and he soon achieved lichdom, becoming a powerful undead mage. His research continued, and he discovered a variant of the Magic Jar spell using a soul gem. He created an ornate 'chain of office' featuring a remarkably lustrous gem, and when the time was right, passed the chain on to his 'successor'.
As each host body begins to age, the lich Guildmaster finds an individual with a physicality he prefers - their actual skills are irrelevant except to further the fiction of succession, as nothing of the host survives - and promotes the person to become a favoured lieutenant. Eventually, he appoints them as his successor, passing on the 'chain of office'. The chain purportedly contains several protective spells intended to aid and guard whomever fills the role of Guildmaster. The old Guildmaster slumps into a coma, and the new Guildmaster proclaims it is the strain of the transfer of the spells which ensure the Guild's success and protect against assassination of the Guild head. In reality, their body collapses into a coma as the lich vacates it - leaving only a soulless husk behind. The soul of the new host is sucked out and devoured by the lich, powering his soul gem and his undead might, as the lich takes up residence in his new host body.
The lich's original body is entombed in state in a hidden location far beneath the city of Bajapur, while he continues to exert massive and unsuspected influence over its underworld.
Thanks to the lich's incredible strength of will, arcane knowledge, and powerful spells, attacks which would kill even heroic individuals are simply shrugged off. Those which manage to breach his arcane defenses are unable to destroy his various soul gems, leaving the lich to reform shortly after, announcing his 'miraculous escape'.
As the Guildmaster does not like to leave Bajapur, he usually sends his closest lieutenant, an individual known as The Brothers Black. Black is the only person to know the truth about the Guildmaster, although even he is unaware of the location where the lich has hidden his true body. The lich bound the souls of four brothers into a single immortal body with a variant of his soul gem, allowing them to exchange control at will, and keeps them in check by threatening to withdraw his spells which maintain them.
The Guildmaster exerts staggering control over illegality within the city, as any criminals who act without joining the Guild are swiftly found and 'invited' to pay their membership dues. Members range from pickpockets in crowds to scheming bards, assassins to thugs, jewel thieves to acrobatic 'second storey men'.