The Mask of Orcus
The Demon Lord has incarnated on this plane.
The heroes attempt to stop Kalarel from opening the rift between their world and the Shadowfell but as Kalarel is consumed by the abomination in the rift Orcus’ succeeds in his plan to walk the material plane. The power that Kalarel wielded in life lends Orcus the energy he needs to inhabit a corporeal body. As the rift is shattered the energy generated tears the fabric of reality and allows the Prince of Undeath to incarnate himself on the template of Kalarel’s soul.
As the rift implodes the heroes are spirited away by a being from a higher plane and all they see is Kalarel being consumed by the creature in the rift. They drift in the space between the planes for an undetermined amount of time and are then transported back to the place from which they were taken. The being in front of them says that it is a messenger sent from the Platinum Dragon. Bahamut intervened and saved the heroes from certain destruction and in doing so has placed them back on their plane of existence 10 years after the incarnation of Orcus walked from the rift.
This being, called Draetoris, now informs the heroes that Bahamut has chosen them out of their time to be heroes in this Age, which has now been proclaimed as the Age of Tenebrous. Tenebrous is the name that Orcus’ incarnation took for itself after it walked from the Keep on the Shadowfell. Tenebrous, with the help of Elder Arantham, has amassed an Army and continues to wage war on the remaining Kingdoms that have yet to fall prey to Tenebrous’ Legions of living and undead alike.
The heroes must rally the remaining free kingdoms and defeat Tenebrous before the world is swallowed by his legions of undeath.