I've met the Rattkin on the Arnika Trynton Road and he wanted me to rob the bank. Where do I find him again?

And a second question. What is the diamond good for?

Hi Valantur!

I guess you've fallen for a bad boy: He won't appear anymore after he brought you into trouble... The diamond can be sold for some money, or give it back to the bank (where it was stolen from) for a little bit of experience. I prefer to bring it back, most people seem to take the money, since at that stage of the game you might need it...

Never forget your towel, cheers
Philipp