Fate/Samurai Remnant - Yagyu Sword Chronicles DLC | Finale
We start off by going into a battle against the Yagyu from before, but now buffed up into a proper boss. We have some very "swordmaster-y" moments for this finale.
Munenori reveals himself to be the real perpetrator and teams up with his fellow Yagyu to face us. Musashi shows up and it becomes a duel of the sword styles. I do like how Munenori can get a grasp of Iori's true nature during this and hints at it. I believe this DLC can be played as early as chapter 2, so it'd be a nice little hint for new players.
After all's said and done, Munenori just shows up again outside Iori's door wanting to hang out like nothing happened. It's great. Then we can use him in the main game as a reward for completing the DLC.