The most challenging campaign.

How does capturing the garrison allow you to kill Agrail?

The point is that if you don't stand in his way, and make sure he can't go around you, he will simply rush past you, hit Nicholas, and that's it – "Mission Completed." By the way, it's not at all necessary to CAPTURE the garrison for this (it's a matter of taste); you can stand right next to him. I captured the garrison because I love large-scale battles (oh, what a bad person came up with the level limit in campaigns; he was wrong:( )

Added after 4 minutes

When I played it for the first time, I also broke through the defense line protecting the Inferno Castle and captured the castle. I had to replay it because the mission became endless. I don't remember if Godric didn't want to leave, or if Agrail didn't appear.

Agrail definitely appears, but I don't remember about Godric; it was probably just a bug