Lore

Director's cut edition of the plot: The lich sets in motion an event that spells doom for the world, you confront and strike down said lich and get info on where his lair resides, the golem guards the lair and the lich's horcrux which is placed in his private study found in said lair, the notes in the study detail his meeting with high priest of azathul - siianov, upon confronting the high priest you learn that the lich was given immortality in exchange for opening a dimensional gate, you find this gate which is guarded by the twin cacodemons Dawn and dusk, awaiting their master azathul to awaken and enter this world. Boom Short and sweet and it fits the production timeline. The lich's horcrux is needed to close the portal after azathul is defeated.