Leveling Up Question

Heya,

Do I need to say 'spoilers'? Is it assumed people here have read the book? If not, 'SPOILERS'

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I was thinking about running this as my first time DMing, though I noticed the intro says the PCs should level up every time they reach a new location and every time they meet/survive an encounter with one of the sisters. Does this mean PCs would potentially level up to 3 after agreeing to steal Skabitha's portrait and then almost immediately level up to 4 when they enter Thither? I feel like I simply must have that wrong.