The Tempus Coin is actually one of seven identical silver coins given to Tempus, the god of war, by the being known as Cyndor. This artifact holds the ability to affect time and space, allowing its user to travel backwards or forwards in the streams of time. Not only can unscrupulous people use it to wreak havoc in the past and take advantage of knowledge from the future, but a single misstep could forever alter the course of history. Therefore, the Coins are closely guarded by chosen Angelic beings loyal to the forces of Good.
Trips through time are exercises in causality. Traveling into the past might set in motion a chain of actions culminating in different major historical events. Conceivably, history could be altered in a way that prompts the Soviet Union to invade and conquer North America. Perhaps the characters can’t even return to their own time because the person who invented the crucial component of the time machine was never born, for some reason. In short, the permutations of cause and effect can be infinitely mind-boggling.
Another potential side-effect of time travel is the alternate reality. The timestream in which time travel is invented continues to exist. Situations that create significant changes or temporal paradoxes serves as the locus or intersection point where realities diverge. The time travelers might encounter worlds very similar to or different from their own.