Ser Derren is a minor noble in the Arling of Amaranthine.
Ser Derren was one of the few nobles in the Amaranthine area who stood against Teyrn Loghain during the Fereldan Civil War. For that reason, the old Arl of Amaranthine, Rendon Howe, made some written promises to give some of his lands to Lady Liza Packton.
These lands also concern the ownership of the southern bridge which apparently has large incomes and was built by the father of Ser Derren.
During the A Day In Court quest, this case appears and Ser Derren supports his position while implying that Lady Liza Packton was involved in Howe's conspiracies. Then the Warden-Commander decides whether this land should be returned to Ser Derren, remain with its legal owner, Lady Liza, or taken from both of them altogether for the reason that the war is costly.
Ser Derren joins the conspiracy against the Warden-Commander if the latter does not choose to give the land to Ser Derren without trying to persuade him, claims the land for himself/herself or allows Varel to decide. However if the Commander uses the persuade option (requires Expert Coercion skill) that he will be repaid somehow in the future, he won't plot against the Wardens even if the persuasion check wasn't successful.
If Ser Derren joins the conspiracy and it is allowed sufficient time to evolve, then Ser Derren will be encountered again in the Throne Room during the quest And You, Esmerelle? where he attacks the Warden and is killed.Alternatively, if the Warden-Commander upholds his claim and returns his land to him, Ser Derren will assist in defending Vigil's Keep during The Siege of Vigil's Keep.