Before you embark on a journey of revenge, dig two graves.
「人を呪わば穴二つ」
"Curses return upon the heads of those that curse.” 呪いは呪うものの頭上に戻ってくる
"Before you embark on a journey of revenge, dig two graves.” 復讐を企てる前、墓穴を二つ掘る
呪う人 - those that curse
呪が戻ってくる - curses return
復讐を企てる - seek revenge/plan revenge/start your revenge
墓穴を二つ掘る - dig two graves