During the first gathering at the Merchant Prince’s Tent, a letter was given to the Wanderlings. After careful decifering, we were able to read the following words:
"This is a key.
Spill blood.
Upon the forest floor,
To call those
Who came before."
This "spell" has been used twice to my knowledge. The first time was outside the Merchant Prince’s tent right after the letter was decifered. The Wanderling who spilled the blood was visited by a spirit.