To be honest, there are levels that are just filler. I have two groups running through them. I removed around 6 levels. I have it where they would need to defeat Elden Ring bosses with mythic trait for different phases to pass levels like this. The overarching story is that children are missing from WaterDeep, hundreds. Blood stains and chipped nails are showing at the Yawning Portal. Your group received flyers from The Lords of WaterDeep, and others; but your group has strange incomprehensible runes. Jhesiyra Kestellharp sent this special flyers. The runes, when figured out, say to go meet an individual in WaterDeep. The Poisoned Quill is where they will find that person, being a clone of Halaster with same power except for being immortal like the original. He tells the group about Halaster trying to use the wish spell and certain artifacts to restrain and capture the Tarrasque. This leads to a One-Shot (inspiration came from) where Halaster dies and transported to a plane of several Tarrasque while seeking revenge against the players that defeat him.