From the introduction: You will be taking part in a Grail War, a battle between teams of two, one Master and one Servant. The Master is the human side, you in other words, who binds the Servant to this world. The Servant is the spirit side, taking the form of a legendary hero summoned from the past. You will use your servants to battle against other masters, until only one remains standing. Only then will you receive the prize. But keep in mind, things are rarely as they seem. Especially with so much on the line.