Get into the telescope assembly bay.

Complete Sunlight.

Film yourself with the film camera.

Read 100 different books in a single playthrough. Adventuring isn't just daring quests, you know.

Take four full long rests in a single playthrough - adventuring's tiring work.

Acquired all skills

Attained the "Shura" ending

Defeated "Isshin Ashina"

Upgraded all Prosthetic Tools to their limit

Upgraded Vitality and Posture to their limit

Attained the "Immortal Severance" ending

Defeated the "Demon of Hatred"