Sekiro: Shadows Die Twice Wiki
A bamboo tube used to see distant objects.

While shinobi aready have good vision, looking through this tube with one eye enables one to see even further.
The Nightjar Ninja serve Isshin Ashina, guarding the rooftops of Ashina Castle, their sharp eyes on the lookout for any would-be trespassers. Not much escapes the eyes of a Nightjar.

Nightjar Monocular.png
Nightjar Monocular.gif

Use to see far away. Unlimited use.

After defeating the Chained Ogre, the item is found atop the building with the broken roof ahead in the Outskirts Wall - Stairway in Ashina Outskirts.