Vecna: Nest of the Eldritch Eye has been released to those who pre-ordered Vecna: Eve of Ruin from D&D Beyond.
It's a small dungeon crawl for 3rd-level characters in Neverwinter's sewers, largely used to seed the idea of Vecna early in a campaign long before Eve of Ruin's 10th-level campaign. Those who pre-ordered can grab it now--it is only available exclusively to those who pre-ordered Eve of Ruin from D&D Beyond.