If you speak with a Caladrius demon, found more prolifically in Shinjuku’s outer area than in the overworld, you may be able to activate the Challenge Quest “Cu Chulainn’s Training”, which can reward you with the Brionac weapon.

You will be challenged to find Setanta, a demon you can find north of Kasumigaseki in the Bugeikan, which is west of the guide in that region.

Boss: Setanta¶

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Winning this battle will net you the Brionac sword. It’s not too great a weapon compared to the Tokkosho, although it does have a 50% chance of dealing three or four hits, which supercedes the Tokkosho in power. It also can hit multiple enemies. I’d probably just sell it, but it’s up to you.