A Witch in Time - S5.E8

When Phoebe must constantly save the life of her new boyfriend, a warlock shows up from the future, claiming that future Cole sent him in order to kill the boyfriend before Phoebe's dies instead. However, present Cole cannot allow this and tries to help the sisters protect Miles, but when the warlock decides to work on his own agenda, by stealing the Book of Shadows, it results in the death of two of the Halliwell sisters. The survivor must decide whether going back to change the past is the right thing to do.