This is located under a statue to the southeast of the entrance to castle nathria raid, right under the cloak.
How do I open the harvest Revendreth?
The Harvest Key is used to open The Harvest at the Abandoned Purlieu in Revendreth. This key was entrusted to Grumbles, who hid it in the area and left a note to remind himself what he did with it. Its location changes based on the note’s current text.