... to talk to Lathaen in his house in the village of Twilight in Shadowspire. It's the westernmost of the 3 houses across the road from the Cathedral of Night down near the SW corner of the village.
He will ask you to return to him Korbu's remains and his sarcophagus. Be careful - the sarcophagus occupies a full half of one of your characters' backpacks! If you want the full spoiler, Select the hidden text between the highlights, below.
Korbu's remains are in the Cyclops' Larder in the SE of Ironsand. His sarcophagus is hidden in Korbu's Crypt, slightly north of centre in Ravage Roaming.