Hallownest Seals are the most common relic found in Hollow Knight, having a total of 17 appearances throughout the Kingdom. Each Hallownest Seal nets you 450 Geo, totalling to a price of a crazy amount of 7650 Geo when all are sold to Lemm.
Here is a brief list of all 17 locations of Hallownest Seals.
1. Given as a reward from Grubfather for saving the 23rd Grub.
2. Found in the Forgotten Crossroads in a secret room on the left side of the well.
3. Found in Greenpath to the southwest of where Thorns of Agony is acquired.
4. In the top section of the room east of Queen’s Station.
5. In Mantis Village in a chest in the room northeast of the Mantis Lords boss fight, can only be accessed after defeating the Mantis Lords.
6. In Queen’s Station up in the ceiling where Willoh is eating mushrooms.
7. In a room to the northeast of the Overgrown Mound, near the Lifeblood Cocoon.
8. In the room connected to the Forgotten Crossroads, hidden in the left wall.
9. Found in the Resting Grounds in the catacombs east of the lift, behind several breakable walls.
10. Given as a reward from the Seer for 100 Essence.
11. To the northwest of Relic Seeker Lemm.
12. Above the King’s Station Stag Station.
13. In Soul Sanctum in the west most room after fighting the Soul Master.
14. In Watcher’s Spire in the room northwest of the Watcher Knight fight.
15. In the south part of Beast’s Den.
16. In a room northwest of the Mantis Lords.
17. Found in Queen’s Gardens to the northwest of the Stag Station.