Thieves guild is a main guild that is located 1 East, 2 South, 2 East, 1 South and 1 East from the Fountain. You can only join it after you have Cleared the Beach.
This page talks about the Thief Guild, in detail. It goes through every quest, and what bonuses you will get.
Quest 1/Startup Quest(.53% total weapon accuracy increase)
They want to make sure you have 100 Dexterity and Intelligence. So train those stats to complete the first quest.
Quest 2(1000+ Fatigue)(3.3% total weapon accuracy increase)
You're told that there is a forge that could be of use to the guild, and you need to go item-hunting for it. You must get a Magic Water, dropped by Gorgon. Head to the Deep Dark Forest, kill some Gorgons in the Pit, and return. It wasn't the right item after all, so now get a Pegasus Wing from Pegasus Scouts at the Mountain top and come back for your second promotion.
Quest 3(2000+ Fatigue)(4.9% total weapon accuracy increase)
This quest requires you to get a Fishblade, which is forged. Kill some Emerald Golem's, Green Slime's, and Dogfish Princess's to create it.
Quest 4(3000+ Fatigue)(6.1% total weapon accuracy increase)
This is a pretty straightforward quest. You must kill Azure Golem in the Deep Dark Forest and return to the guild with an Azure Diamond.
Quest 5(5000+ Fatigue)(7% total weapon accuracy increase)
You must find the Quetzalcoati and find a new pair of Serpantscale Pants. Quetzalcoati is located at the bottom of the pit in the Deep Dark Forest.
Quest 6(7000+ Fatigue)(7.7% total weapon accuracy increase)
Now is the time for some grinding. You're required to slay 500 Winged Archer's in the Sky Castle. She resides on all 3 floors of the castle so it's your choice where to do it.
Quest 7(10000+ Fatigue)(8.3% total weapon accuracy increase)
You must get an Ornate Fascia from Salena. She can be found in the Great Plains, quite near to the Plains teleport location. Then return to the guild to receive your next promotion.
Quest 8(15000+ Fatigue)(8.8% total weapon accuracy increase)
Another slaying task. This time you need to kill 1000 Fire Golem's in the Lower Volcano. It dwells on 2 volcano levels, so it's another choice for you.
Quest 9(25000+ Fatigue)(9.3% total weapon accuracy increase)
You need to get a Black Pixie Dust from the Black Slime which dwells in the Vast Desert. Go to a spot near any desert Oasis to find it. Return with the Dust to the guild to get your next promotion.
Now, with all 9 quests completed, you can now forge the guild's secret formula, the Master Bracelet. Implemented 6/6/2022
Quest 10(40000+ Fatigue)(9.7% total weapon accuracy increase)
Once you have forged the bracelet you have one final quest to complete. Go to the SE corner of Olde Towne and defeat Rob Err in the abandoned shop.(Quest starts in Shillatown forge)
