Mister Mushroom

Mister Mushroom is a talking mushroom found through various locations in Hallownest after the player has progressed past a certain point. His babbles can be heard from a distance, guiding the player to his location. Finding him in all seven locations rewards the Passing of the Age achievement and a secret cutscene after the player has beaten the game.

Riddle Tablet
The Riddle Tablet is a glowing glyph in a secret room at the bottom of Kingdom's Edge. When the player reads it with the Spore Shroom charm equipped, the tablet reveals a poem, which gives hints to the locations where Mister Mushroom can be found.

Locations
After reading the tablet, Mister Mushroom can be found in 7 different locations. Each one must be visited in the order described on the tablet.

1) Fungal Wastes 2) Kingdom’s Edge 3) Deepnest 4) Howling Cliffs 5) Ancient Basin 6) Fog Canyon 7) King’s Pass

Trivia

 * According to Mister Mushroom's dialogue, he has witnessed many more civilizations before Hallownest, and all seemed to have been started by Wyrms.
 * Mushrooms, and fungi in general, are known in real life to live to very old ages and span across vast networks (the oldest living mushroom is over 2400 years old and 3.5 miles long). Mister Mushroom could very likely be a sort of god in the Hollow Knight universe, living and seeing through other mushrooms.