From the back cover:
This is a sequel to Out of Body, Out of Mind. This first adventure is not required, but is recommended to take full advantage of the alterante setting of Tirna'cel.
The legend of the realm of Tirna'cel is well known. Once remembered for its power-mongering warlords, homunculi-wielding sorcerers, and archfiends from the deepest pits of the Abyss, Tirna'cel has become a powerful and welcoming land over the last 600 years, due in no small part to the efforts of the warlor Tirna'gael, a member of the land's founding nobility.
But your party knows the truth. You ventured into the Tomb of Tiran'gael some six months ago, and therein discovered that the fallen warlord was actually little more than his brethren: a petty, avaricious power-monger. Possessed by a rival force which also desired the kingdom - a rare incorporeal demon called a nescent, which inhabited his body and augmented his already incredible power to unforeseen levels - Tirna'gael sought to overthrow the ancient warlords only to replace them.
Through Tirna'gael's augmented body, the nescent overwhelmed and eliminated the other warlords, eventually dominating the realm alone. But before it could consolidate its rule and throw Tiran'cel into an even darker age, the secret of its power was discovered by the paladins of Garadon, a virtuous cell of justice and light. The paladins lured Tiran'gael into the center of his own fortress and trapped him there within a powerful antimagic field. His body withered and died but the nescent remained dormant, waiting for its chance for revenge.
During your visit to Tirna'gael's fortress-tomb six months ago, you accidentally released the nescent from its prison. That enemy has since been vanquished - at least for the time being - but the memory of your pitched battle against it still lingers as you approach the original chapter house of the Order of Garadon.
You have been summoned.