In Sengoku Japan, Onimaru is a righteous swordsman affected with amnesia after an attack on his village slays his master then must figure out his relation to him.
I like where you’re going with this, but your logline could use some improvement. I wouldn’t mention the ‘relation to the master.’ I would say:
After a viscious attack on his Japanese village leaves him with amnesia, Onimaru a swordsman, seeks to recover his memory and find his master.
Don’t mention the master is dead. Leave this as a surprise at the end. So when he finds out his master is dead, he can go beast mode on the big boss.