Both Kelemvor and Cyric, along with Midnight who is the current Mystra, were part of an adventuring party during the Time of Troubles. At some point Cyric kills Kelemvor. Cyric and Midnight became Gods, and I don't recall what the fourth member of their party did.
Kelemvor was hidden inside Cyric's sword, Godsbane (avatar of Mask). Cyric searched for Kelemvor's soul, could not find it, and broke his sword in frustration, releasing Kelemvor. Blah blah blah, something about Kelemvor's bravery against Cyric caused the people of the fugue plane (or whatever it was at the time that houses the dead) to want Kelemvor to rule over them instead of Cyric.
I recently read a wikipedia entry on it and that's what I came away with.
Am I right to presume that Kelemvor's replacing Cyric as the god of death and Bane's return are what heralded 3.5e? Or were these earlier, around 3e?