Kill the Master Thief

Quest Giver	Sjoring Hard-Heart in Vivec
Prerequisite Quest	Remove the Heads of the Thieves Guild
Reward	promotion to Master, +15 (Fighters Guild)

Finally Sjoring Hard-Heart in the Vivec, Guild of Fighters wants you to kill the Master Thief of the Thieves Guild.

Gentleman Jim Stacey can be found at Vivec, Simine Fralinie: Bookseller in the Foreign Quarter Canalworks. He is behind the locked (65) door in the back of the bookshop. Frenzy Humanoid or Taunt can be used to force him to strike first, in order to avoid any negative consequences when killing him. Attacking him outright will incur a Bounty on your head and cause Crazy-Legs Arantamo to become hostile.

When you return to Sjoring Hard-Heart, he will immediately attack, arguing that you are a threat to his position. Defeat him to become Master of the Fighters Guild.