Party level please? And has there been no, a little, or a lot of planar travel/excursions?Thanks everyone, yes this is 5e. The setup is they are approaching a castle and the knight (actually a samurai in this case) comes out to meet them. Here is a draft version of the boss.
I lifted the idea from he video game Sekiro.
I see CR as a flexible concept so not wedded to it. Flying creature. Hmm. Good idea but too late as I ended the session with the swine shogun emerging from the castle and challenging them to a fight. But a Japanese/Chinese dragon may have worked quite well in retrospect.
Party is level 9 and most likely x6 PCs. So will most likely beef up the shogun's HPs if nothing else. One of the reason I gave him mobility and a lot of knockdown is to better deal with so many PCs.
Zero planar travel.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.