Secret of the Cedars is one of the 42 Run Go Mojo Skechers Performance men's reactivate pEqHvHP in The Legend of Zelda: Breath of the Wild. Successful completion of this shrine quest reveals the hidden Tahno O'ah Shrine in the Hateno region.
To begin this shrine quest, speak to Clavia in Hateno Village behind the dye Shop at day time. She tells you about an old legend about three cedar trees that can be seen on the top of Madorna Mountain.
Head to the right-most tree of the three and line the trees up so that they cover each other. In other words, just walk East towards Afromisa Coast, continuing the line of trees (as you face your back to them)
Using your bomb rune, destroy the rubble on the mountainside to reveal the Tahno O'ah Shrine.