Gothic II/Khorinis

Canthar's Favor
Whether you accepted Canthar's offer or not, he will now proceed to blackmail you as soon as you come within vocal range. He will demand that you get Sarah the weapon merchant arrested. If you met him with your farmer clothes on before, he'll have assumed you are a farmer and you can avoid doing any dirty deeds or paying the 500 gold.

If you were't in farmer's clothes, start trading with Sarah and "sell" her the incriminating paper. After that, go to lord Andre and pocket the bounty. Canthar will give you a weapon and this is the end.

Of course you can also get Canthar arrested just by talking to Andre. Sarah will also give you a weapon, however in chapter 2 Canthar returns from jail and makes all merchants hostile towards you. You can reverse that by giving him 500 gold. Or you can kill Canthar before chapter 2 by attacking him in his jail cell and luring him away from eyewitnesses (strangely nobody will mind him leaving the prison).

Information for Garvell
Garvell is the paranoid boat builder. You can obtain the first two bits of information he needs by giving beer to Ragnar the militiaman by the gallows. You get the final piece of information from Lord Hagen once you agree to go to the Valley of mines.

The Experiment
Ignaz the alchemist has a hut behind Matteo's shop. Accept to perform the experiment and he will give you a scroll.

If you do a little research, you find out that the ideal target is Valentino from the upper quarter. He spend most of the night in the tavern near the temple and leaves at 4 in the morning. You can beat him up behind Bosper's shop. Once you do that, use the spell and return to Ignaz.

You should also talk to Regis. He will give you a ring that you can pass on to Coragon the innkeeper.