The adventure can end one of several ways. Ideally, it concludes with the adventurers witnessing Claugiyliamatar’s body becoming possessed by Chardansearavitriol’s spirit and the completion of all the The Quest Board. By then the characters should be 11th level. Give the players a chance to wrap up loose ends before declaring the adventure over. For example, the characters might need to return to Leilon to collect an outstanding reward or notify the town council of Claugiyliamatar’s possession. The story doesn’t have to stop there. If you play Divine Contention, the next adventure in the Beyond Dragon of Icespire Peak series, the characters will have final showdowns with the cults of Myrkul and Talos that decide the ultimate fate of Leilon.
If the worst happens and the characters die, their adventure is also over. You can let the players roll up new characters and pick up where the last ones left off, or you can also start the adventure over with new characters, tweaking quests as you see fit to surprise your players who might otherwise be familiar with them.
You might expand the adventure by inventing new quests and new locations. Several other D&D adventures, Out of the Abyss, Princes of the Apocalypse, Rise of Tiamat, and Waterdeep: Dungeon of the Mad Mage, take place in the Forgotten Realms and include challenges for characters of 11th and higher levels.