Seamus Diggory is a soldier barricaded inside a locked wooden shack deep in the forest of the Cursed Hollow, during Chapter 6, Punishment. You reach him by heading past the army camp with the cannons, crossing the flooded riverbank, and climbing the rocky trail up into the dark trees. He will only speak through a peephole in the window, and he wants blood.
Where to find Seamus Diggory
After passing the army camp and the Lantern beside the river, wade through the flooded area and ascend the rocky slope into the forest. On the climb, take the Indestructible Shell Amber from the red chest beside the cliff before continuing.
Past the carts, an orange glow shines through the window of a log shack. Approach the shack to start talking with Seamus. He offers two responses, “Alternate measures?” and “Desertion?”, then asks you to bring him blood. His quest entry says he says sunlight makes him ill and directs you to get an untainted sample from the nearby grove.
How to complete The Curious Case of Seamus Diggory

Only the recently dead pig gives a usable sample. The plagued animal on the grass will not work.
Reward and how to know it worked
Give Seamus the Blood-Filled Bottle to trigger “THE CURIOUS CASE OF SEAMUS DIGGORY / QUEST COMPLETE”. The bottle is removed from your inventory, and you receive a Ferrofluid Flask Power Upgrade in its place.

Continue along the route into a canyon, then pass through a short tunnel to reach the second flooded village. In the village square, use the Lantern to upgrade the Ferrofluid Flask.






