As soon as you bring the assassin's scroll to Heidric, he changes his attitude towards you, and gives a new quest. You have to find a spy in Ganestor.
Looking for the spy
You learn a keyword from Lord Heidric: spying.
Heidric suspects Lord Dunbar to be the spy. He lives in a house in the north of Ganestor. There is no evidence of his spying.
The most classic place to start investigation is the tavern. Chitchat with Smitty the innkeeper, and he'll tell you a keyword (sneaking) for Barton. The resourceful serf reveals that a guard sneaks out of the castle at night: that's our man!
BOSS: the spy
Get to Heidric's castle gates and wait for the night. You may notice that the "candle of life" slowly refills if you stand still.
Early at night, a guard will come out. Ask him about spying, and combat will start. Use the blue "ice" scroll to make things easy.
When the guard is defeated, pick up a note he dropped.
Bring the spy's note to Heidric, and the plot will evolve a little bit. You knew already that Dariel is an evil ruler, but the news are that the prince of Ganestor, son of Heidric, has been kidnapped.
- Note that Lord Dunbar is not involved at all in the spy problem: there's no evidence against him.