Sure.
Know about Stargate, then?
Right, Lionheart is set back in the time of the Crusades when Richard I (English king) was fighting Saladin in the Holy Land. 1300's I think.
So, Richard captures a town and wants to blackmail Saladin. Saladin does not capitulate. One of Richard's advisors suggests suggests that the 2/300 prisoners are all killed.
Unbeknown to Richard this completes some kind of Ritual, which causes the 'Disjunction' an event releasing magic into the land of good and evil origin.
Richard and Saladin fight together and beat back the evil and close the portal where the magic is coming from, but too late to stop the magic already released.
Then the ordinary timeline is twisted, really. Powerful leaders combine with magic. Mongols attack and force their way to Italy which never recovers, crusades against evil Storm Dragons, that kind of thing.
So really, it's an alternate timeline kind of thing.
More timeline info. for Lioheart is here:
http://www.gamebanshee.com/lionheart/timeline.php