Empire of Storms Theory
The merchant Celaena met in The Assassin and the Dessert was Lysandra’s father.
So in The Assassin and the Dessert, Celaena meets a mechant who sold twenty years of his life for some spider silk.
In Heir of Fire, Manon talks with a Stygian Spider who gloats about taking 20 years from a shape shifter merchant.
Lysandra’s mother wasn’t a shifter which meant it had to of come from her father.