Invisible Ink is a Document in the Rise of the Tomb Raider: 20 Year Celebration DLC Blood Ties. It belongs to the Lord Croft's Journals set and contains the description of Lara's birthday expedition in the basement of the Manor that her father and Winston organized.
I've created a clever little map for Lara's Birthday Expedition. I used the INVISIBLE INK Amelia picked up for me in Morocco so many years ago. At the time I remember being so annoyed by that rather aggressive street vendor, but she just smiled in her sunny way and paid him without a second thought. I assumed it was a tourist scam, but it actually works.
Turns out it requires a very specific kind of vellum, and a heat source to illuminate the ink after it's dried. It should prove a nice challenge for Lara to puzzle out.