The Elder Scrolls IV: Oblivion Guide - Fighter's Guild
Speak to Modryn Oreyn after the Water's Edge incident; he concludes that the only way to stop the Blackwood Company is to destroy the Hist Tree, the very source of their power. You need to travel to Leyawiin, enter the Blackwood Company building, locate the Hist Tree and destroy it. Oreyn warns that leader of the company, Ri'Zakar, will have the tree well-guarded, so don't expect a warm welcome. Prepare well before entering the Blackwood Company hall, as you will be attacked by the guild members in the hall almost instantly.
First, you'll need to kill Ja'Fazir and retrieve Jeetum-Ze's Room Key from him. Jeetum-Ze holds the key to the Ri'Zakar's room, so you'll have to kill him first. Jeetum-Ze's room is on the second floor of the main hall. Kill Jeetum-Ze and remove Ri'Zakar's Key from his corpse. Ri'Zakar's room is on the top level of the hall. Make your way up to Ri'Zakar's room, kill him, and remove the 'Blackwood Co. Basement Key' from his corpse.
Kill Ri'Zakar, as he holds the Blackwood Co. Basement Key.
When you have this key, head back down to the main floor and locate the door marked 'Blackwood Company Hall Basement.' The Hist Tree is through here. It would be wise to save your game before heading through the door.
Once in the basement, you'll need to destroy the machinery surrounding the Hist Tree. First, kill the two Blackwood Company members in this room. Then, head to the side of the room opposite the door used to enter the basement. Pick up the two loose pipes on the table here. Then, approach the nearby Sap Pump and press the A/X button when prompted to jam one of the pipes between the cogs, effectively breaking the machine. Approach the Sap Pump on the opposite side of the tree and do the same.
Destroying both of the sap pumps will cause the Hist Tree to go up in flames. With that done, head back up the stairs and start toward the front entrance. You'll run into one more enemy before leaving guild hall. It's Maglir; you have no choice but to kill him.
Sabotage the two sap pumps by the Hist Tree to set it aflame.
Return to Chorrol, speak to Modryn Oreyn and tell him of your success. He'll give you the Helm of Oreyn Bearclaw as a reward. Next, head to the Fighters Guild hall in Chorrol and speak to Vilena Donton there. Once you've explained what has happened, speak to her about your advancement to be promoted to Master of the Fighters Guild!
Select the 'Duties' dialogue option and appoint Modryn Oreyn as your second-in-command. Speak to Modryn Oreyn, who is unconvincingly reluctant to rejoin the Fighters Guild as your second-in-command.
As the Master of the Fighters Guild, you are able to assign duties to the guild once a month. Speak to Modryn Oreyn, and you can choose to have the guild focus on recruitment, contracts, or both. Choosing to focus on recruitment yields a better chance of having items delivered to the Guildmaster's chest on the top floor of the Fighters Guild hall in Chorrol. Choosing to focus on completing contracts will have money delivered to the chest instead, while focusing on both yields will yield items and a small amount of gold.