The EX Strange Mask Rumors quest in The Legend of Zelda: Breath of the Wild is a DLC-exclusive adventure that sends Link on a hunt for a Korok Mask that has the power to alert its wearer when a Korok is hiding nearby.
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EX Strange Mask Rumors Quest Guide
How to Start EX Strange Mask Rumors

To begin the EX Strange Mask Rumors quest in The Legend of Zelda: Breath of the Wild, head to the Woodland Stable next to Pico Pond, which is south of the Woodland Tower.

Inside, read the book on the table, Super Rumor Mill EX V1, to learn the first clue about the location of the mysterious mask:
“Some say the treasure chest containing this mask is hidden in a tree hollow in a forest that makes people lose their way.”
EX Strange Mask Rumors Walkthrough
Finding the Korok Mask

For this clue, travel along the road northwest of the Woodland Stable, going through the Minshi Woods, to reach the Lost Woods.
Once in the Lost Woods, follow the embers on the torches until you reach the two that are side-by-side. Grab the torch and light it. Follow the embers until the wind changes.

Instead of following the embers to the right, continue straight a short way, then look left and activate Magnesis to find the chest in a tree hollow. Use Magnesis to pull the chest out and open it to get the Korok Mask!

Those fun-loving Koroks inspire this Korok Mask. It shakes when a Korok is hiding nearby, making hunting them easier.

