What need would you have of a potion that would make the one who took it tell you all the truth. You Shadowhunters have no secrets among each other, I hear.”
-- Mother Hawthorn (Cast Long Shadows)

Importance: Matthew is in the Shadow Market when he encounters Brother Zachariah, and a faerie woman has potions for sale including a truth potion. The woman's question shows one of the stereotypes the Downworlders have about Shadowhunters – that they never keep secrets. Matthew does not buy the potion at that moment, but he returns for it later and gives it to his mother in the hope of finding out the truth about his parentage.

If I were to tell my parents the truth about myself, if I were to reveal who I really am, they would despise me. I would be friendless, cast out, alone.”
-- Ariadne (Every Exquisite Thing )

Importance: Anna has just learned that Ariadne and Charles will begin courting with plans to marry...