"Kilton's handmade Horriblin headgear. The big nose and smiley mouth look just like those of the real monster... Equip it to blend in with Horriblins."
The Horribin Mask is a piece of armor found in Tears of the Kingdom.
Tears of the Kingdom
A Horriblin Mask has a base defense of 3, and cannot be enhanced. This item can be traded with Koltin at the Fang and Bone, which only appears at nighttime in select locations, for 4 Bubbul Gems when offered, and will cost a culminative total of 29 Bubbul Gems. It allows Link to blend in with various Horriblins, who will inspect Link if he draws near whilst this item is equipped. If Link sells the Horriblin Mask to a shop vendor, he can repurchase it from Cece in Hateno Village for 4,000 Rupees after completing The Mayoral Election side adventure.
All types of Keese are immune to the effects of Kilton's masks, so the disguise provided by the mask will be rendered useless by their presence.