The Tunic of the Depths is a piece of armor from Zelda: ToTK; it offers a measure of defense ... 3 pieces of the set offers the bonus effect "Gloom Attack Resist", which provides 4 Gloom Hearts.

Poes can be found scattered throughout the Depths, the underground area of TOTK's map. The Depths can ... be wearing conical caps that resemble the Gloom-resistant armor set that they sell ...

Gloom Resistance is an effect in Zelda: Tears of the Kingdom that offers 1 Gloom Heart at the base level. It can be obtained from Armor and Food.