It can be acquired from the map table in Heroes Guild. In the Execution Tree quest you need to ensure that a bandit reaches the execution tree in Headsmans Hill.
There is also an evil variant of this quest, named Execution Tree Rescue, where you join the bandits and save the prisoner from the guards.
More details about the optional quest can be found in the following Fable Anniversary walkthrough.
Renown: 200 + 260
First you need to teleport yourself to the jail outside Bowerstone. The guards that are transporting the prisoner ask you to accompany them, because they think that the bandits friends might try to rescue him.
You will fail the quest if the bandit or the guards are killed, so try to stay close to them because bandits ambush you all the way to Headsmans Hill.
You can use the Slow Time spell, but try not to use area of effect spells or you may kill your allies. When you enter the Headsmans Hill area you are ambushed for one last time.
Use the same tactics to defeat them, and then watch the cutscene.
The quest ends at this point and you collect your reward.