As soon as you touched the Tablet of Truth, it informed you that the king's assassin is in Lankwell. After you collected the reward for returning the Tablet to Dariel, it is time to start the new quest.
City of Lankwell[edit]
Lankwell is a city south of Eralan. No lord rules it, and its inhabitants are "thugs" rather than "serfs".
Talk to Baebon in the tavern, and ask him about an assassin. He tells you a new keyword: Black Asp. Check all the houses and ask another thug about this: he will inform you that the Black Asp lives south of town, across a bridge.
While in Lankwell, you can purchase a magic axe (unless you did it already) for 95 Gold Pieces.
Leave Lankwell to the south. The map shows a bridge to the south-east. Cross it and follow the bare earth path. Talk to the Black Asp: he will tell you the truth about King Valwyn and the Lord Dariel. He also writes it on a scroll you can bring to Lord Heidric in Ganestor.
From Lankwell to Ganestor[edit]
In order to get back to Ganestor, you can of course follow the road: it is longer, but you can lodge (heal & save) at three inns along the way (in Eralan, at the Last Hope Inn and in Hampton). Otherwise, you can follow the river on either side.
In Ganestor, the castle gates are still closed. You have to use the secret way again. If you killed a guard during the previous quest, he will not respawn.
Give the scroll to Heidric to complete this quest, and he promptly sets you on the next one.