When looking at several places online, I've heard people talking about getting the "mark of the beast" in this game. Does anyone know how?

I may be wrong, but I don't think it is possible. At least, Cardinal Truncquillius in Act II offered me such a thing ("Come closer and let me mark you by this infernal brand that'll become your new symbol!"), but in the end he laughed at me ("Muheheeee! You fool! Did you really think I would let you enter there where only the chosen ones may enter?").