The adventure ends when the players have completed both the follow-up quests and put an end to the rival cults threatening Leilon. The characters should now be now 13th level. Give your players a chance to celebrate their victory: a feast in Leilon, or the trust of Lord Protector Neverember. If the characters saved Leilon, the party is remembered forevermore as their saviors.
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 players who might 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 level.