Tobio's Hollow is a valley full of dead trees south of Dueling Peaks that is home to a sleeping Hinox and contains a Bottomless Bog north of where the Hinox can be found. The bog contains two hidden Koroks.
Swamp in the Valley of Dead Trees
According to Misko's EX Journal 2, the helm of the self-proclaimed King of Shadows stolen from Hyrule Castle by the bandit Misko during the Great Calamity can be found in "the swamp in the valley of dead trees, south of Dueling Peaks." which describes the bog located in Tobio's Hollow. During EX Treasure: Usurper King "The Champions' Ballad" DLC side quest, an EX Treasure Chest containing the helm can be found buried on a small island in the bog between two dead trees.