Path of Stars
From Hades 2 Wiki and Guides
Useful Pages:
Nocturnal Arms, their Aspects & Upgrades
All the Boons
Hades II Guides
Path of Stars are used to upgrade 
Hexes. By default, each Path of Stars gives 3 upgrades. Hexes selected with the Half Moon get +1 upgrade to the first Path of Stars, and Full Moon gives +2 upgrades. The Moon Beam Keepsake also grants additional upgrades to the next Path of Stars.
In the Fields of Mourning, you can find smaller Path of Stars as a Minor Find. These only grant 1 upgrade each.
Each Hex has three possible upgrade trees, with the exact upgrades randomized each run.