Sinclair
Talk to any of the guards around Sinclair Mansion to find out that Lord Sinclair was murdered, and they have no leads as to who or why, and they ask for your help. Enter the mansion and enter the first door to the east, then north through the next door.
Take the Pungent pot. It seems Lord Sinclair was drinking from it before he was murdered. Take the Criminal's Dagger, and it doesn't seem good enough to have stabbed someone. Investigate the smashed window on the east side of the wall to find a piece of thread caught by a nail.
Go outside the mansion, and on the west side, go into the shack and search the sacks. Take several sheets of fly paper.
Evidence
Go back in the mansion, and search all the barrels (2 downstairs, 4 upstairs) for all the silver items. When you have them all, go into kitchen with a pot, and fill it up with flour. Use the flower on the criminal's dagger.Use some flypaper on the dagger to get an unknown fingerprint. Then, use flour on the different silver items and flypaper on them to get finger prints. Use the known fingerprints with the unknown to find out if they're a match or not.
Once you find a matching print, head down to the Seers' Village bar, and talk to the poison Salesman. Ask him who he sold the poison to, and he will tell you each of the children.
Head back to the mansion, and talk to the person that matched the finger print to. Ask what they used the poison for, and they'll tell you one of the many things around the mansion. Go to investigate it to find out they didn't use the poison on it.
Solving the case!
Talk to any of the guards, and tell them you know who the killer is. You will be congratulated and rewarded.
