When he asks you to strike him down, walk away instead. Once you have Great Slash and have rested at a bench, he is at Sheo's hut in Greenpath; talk to him there.
Climb the Pantheon of Hallownest and beat Absolute Radiance at its top, without having given the Godseeker a flower. With 112% on the save this ending also earns Pure Completion.
Keep playing the cycle 2 save: banish the Troupe, strike down the Nailsmith and reach 100% before 20 hours.
Brumm: break the Nightmare LanternHidden spoiler – activate to revealQuest stepMissablePoint of no returnDLC: The Grimm TroupeNG+1Minor spoiler
Location
Howling Cliffs · Nightmare Lantern
Why you can lose it
Leaving the lantern room after Brumm's talk there, before breaking the brazier, makes him vanish for the save.
Point of no return
Leaving the Nightmare Lantern room after Brumm's talk there.
Summary
Run the Grimm Troupe ritual past Troupe Master Grimm, then find Brumm in Deepnest's Distant Village. He meets you again at the Nightmare Lantern; strike the brazier and do not leave the room first.