12月なのでクリスマス要素とか思ってたのが聖夜→聖属性→聖騎士
吸血鬼に強い得物は多分樅の木

Since it's December, I put in some Christmas elements: saint night -> saint type -> saint kight.
seiya (literally "saint night") means "holy night"; seikishi (literally "saint kight") is normally referred as "paladin" in English.
Fir trees are effective against vampires, I think.