Nasaug is the leader of the Canim warrior caste. He is shown to be very intelligent and a very capable leader of his people.
Nasaug first appears in a Canim war camp outside of Elinarch. Tavi has recently successfully defended a raiding party's attack on Elinarch and visits the camp, taunting Sarl into appearing. Tavi makes several attempts to taunt and discredit Sarl in front of his war camp. During the talks, Tavi offers to let the Canim come to the wall to recover their dead. Before the offer is accepted, Sarl attacks Tavi and drives him back to his defenses.
To save face, Sarl orders Nasaug to assault Elinarch. The First Aleran is holding the wall well when Nasaug calls Tavi out to talk. Nasaug reminds Tavi of his offer to allow the Canim to collect their dead. Tavi allows them to do so and invites Nasaug to play a game of ludus while they're waiting. Nasaug accepts. During the game, one of Sarl's assistants approaches, asking why Nasaug has ceased his assault. Nasaug bites the messenger's hand off and says he's in charge of when the attack takes place. Tavi shows himself to be a strong tactician and eventually defeats Nasaug using a technique he learned while playing with Ambassador Varg. When Nasaug asks him about it, he informs the Canim leader than he learned it from the Ambassador who is still alive, but in prison.
Nasaug then shows the real reason he has paused combat - not only was he collecting the dead, he was also killing time. Sarl's assault had started well before the sun had set. The collection of the dead took up time. Nasaug knew that Alerans can't see in the dark and would be at a disadvantage during a noctournal battle. The ensuing attack is overwhelming, and Tavi is forced to give up the wall and most of the bridge.
Before Nasaug can completely overwhelm the Alerans, Sarl ceases the assault. Nasaug's success would put him in strong favor over the Canim and Sarl wanted to maintain control. To that end, he ceased the assault and uses a raiding party to show that the Canim can't beat the Alerans without the ritualist class. When Sarl pushes his ritualists forward, they're met with a deadly trap Tavi has come up with. Sarl and most of his ritualists are killed in the trap.
After Sarl's failed assault, the Canim withdraw. Tavi assumes that Nasaug's push forward would cause Sarl to stop it and push his own forces forward to maintain their appearance of strength, which eventually got them killed. From this perspective, Nasaug appears to have done everything to force Sarl into this position so that Tavi and his group would kill Sarl.