Gothic II/Thieves' Guild

Joining
Before all you must know that you can not join the Thieves' Guild if you are already a novice at the Monastery of Innos or a member of the militia. If you are a mercenary you can join them. That is why is very important to join the Guild before you join the militia or the fire magicians, you can become a city guard or a novice after you join the Guild, and the Guild members won't have a problem with this.

In order to join the Thives' Guild you must complete 3 quests involving 3 secret members of the Guild. This members are Rengaru, Naguru and Halvor.

Constantino's Ring
You receive this quest from Cassia once you locate the thieves' guild and decide to join them.

The ring is in the locked chest next to Constantino's bed. Wait until he is asleep and then sneak into his room. You need to have both the Sneaking and Lockpicking skills to accomplish this.

The Blood Chalices
Cassia gives you this job one you join the thieves' guild.

These are the locations of the six chalices:
 * Lehmar, the money lender, has one in the chest next to the house entrance (he does go to sleep eventually)
 * One is in the house next to Salandril's shop (the key is on the fireplace). This one is a bit difficult, because even after you bribe Wambo, the other guard almost never seems to leave. But you can easily snatch the key during the day and sedate the guard with a spell.
 * Upper floor of the Paladin headquarters. You can take it any time after you gain entrance to Lord Hagen.
 * Gerbrandt's house, in the chest near his bed. The paladin never leaves but he does not see as well as you would expect.
 * Valentino's house, one of the lanterns opens a secret room and you need his key to open the chest.
 * Judge's house. Press the wall shield on the upper floor to open a secret room. You need the judge's key to open it.