Some people may think that mercy shouldn't be a part of justice, but our narrator disagrees and doesn't think they have to be irreconcilable.
All of Mercilla's subjects think she's a very just and merciful queen, and Arthur, Artegall and everyone else is convinced that Duessa is guilty and should be punished (although we never actually hear what that punishment is).
After the Duessa trial, Artegall and Arthur stay longer at Mercilla's court enjoying the queen's company.
One day young men come to the court to ask for help for their mother, whose land has been invaded by a terrible tyrant.
The mother's name is Belgae, and she had been a great lady with seventeen children before this tyrant had killed twelve of them and left only five.
This tyrant was a kind of monster with six arms and six legs who was believed to be the son of giant and from a family of purple people.
The tyrant was called Geryonco and he fled to Belgae's land after his father had been killed.
Since she had recently been widowed, Geryonco convinced Belgae to trust him, since she needed help, and eventually gave him the power to rule.
Once he gained power, he became a terrible ruler, giving up her children one by one to a monster and oppressing the land.
So her last two sons had come to Mercilla for help, and Arthur asks the queen if he can go with them to help.
The queen agrees and Arthur goes off on his quest, leaving Artegall to go off on his own quest himself.
Soon Arthur arrives at the land of Belgae, and he finds her hiding from the tyrant alone in the wilderness.
She is overjoyed when she sees her two sons coming toward her and thanks Arthur greatly for coming to their aid.
Arthur feels bad to see her in this sad state and comforts her and urges her to go somewhere more comfortable.
She replies that she has nowhere to go that isn't under the tyrant's power, but Arthur promises that they will find somewhere or he'll force somewhere to take them.
They arrive soon at a city that used to be the queen's until it was taken over by the tyrant.
He built a luxurious chapel in front of the city's castle dedicated to his idolatrous religious beliefs.
Underneath the altar of that chapel lurks a horrendous monster and the chapel is guarded by a fearsome soldier.
Although Belgae tries to dissuade him, Arthur rides up the castle and calls upon the guard, who rides up to fight.
They begin fighting and Arthur ultimately triumphs, stabbing the solider right through his shield.
Three other knights then ride up to challenge him as well, but after Arthur handily dispatches one of them, the other two flee in terror.
Arthur follows them and kills them both, finally winning entrance into the castle since none of the other residents are brave enough to face him.
And so Arthur triumphantly leads Belgae and her sons into the castle.