His role is to defend and enforce specific covenants and rules prescribed by the treaties of Wyrmroost and to slay any dragons that go against them. He has resided in his dark and lonely dominion since Wyrmroost was founded, and is of the opinion that dragons should have been thoroughly exterminated rather than confined to dragon sanctuaries. He has a dullion bull mount named Umbro. He is so named for his dire and pessimistic perspective of life in general.
Ryland senses the declaration of war made by Celebrant and is tipped off by Lord Dalgorel where to look for Kendra, Seth, Tanu, and Eve, and arrives in time to save Seth from two Dragons, Chiro and Numrum, killing them though he learns Kendra has been captured by the Dragon Jaleesa and taken to Celebrant.
As Amulon speaks for the taurans, and a score of them aided him in his crime, the Somber Knight revokes his claim to the Herdland territory, along with access to the thoroughfares reserved for friends of Blackwell Keep. His new domain will encompass the Barrows, Lackluster Woods, and the Adjoining Meadows, a domain of haunts and shades.
Amulon is threatened to reach his new domain promptly, for anywhere else on the preserve will he be found in extreme violation of trespass, with prejudice, and his kind will be hunted anywhere else they go, fair game to any creature.
Furthermore, the taurans still at the Herdlands are also trespassing, and should they linger for more than a day, their lives will be forfeit. Once they have departed, reentry is of course barred. And then the Somber Knight pierces the High Road with his sword, declaring Amulon and his people will be rejected by the road's protections, and indeed a defiant rumitaur is injured by and hurtled off the road when he tries to defy the Somber Knight. As Celebrant has declared war and had Kendra kidnapped, the Somber Knight, after making sure Seth, Eve, and Tanu will be fine, goes to Skyhold where he strips Celebrant of his caretakership and is attacked in retaliation by Celebrant's dragons. Before the Somber Knight can rescue Kendra, Raxtus frees her and spirits her away.
The Somber Knight just barely manages to escape but loses an arm up to the elbow and a leg up to the hip while his mount Umbro is bisected. The fact he didn't bleed led Tanu to suspect the Somber Knight may be undead.