You have not completed and are not venturing in Tomb of Annihilation.
You control a Treasure token or a Gold token.
({B} magic symbol) available.
Steps
Cast
Acererak the Archlich by paying ({B} magic symbol) .
Oltec Matterweaver triggers, creating a copy of a Treasure or Gold token you control.
When
Acererak the Archlich enters the battlefield, its first ability triggers, returning it from the battlefield to your hand, and venturing into Lost Mine of Phandelver.
Activate a Treasure token by tapping and sacrificing it, or a Gold token by sacrificing it, adding ({B} magic symbol) .