There are a total of three Relics that use the Belt slot, all of them increase the maximum number of slots for quipping Relics wich make them unique from the rest of Relics.
Location: Underwater Chest
Abilities
List of al lthe abilities of the Drowned Belt
Load Capacity
Increases the maximum number of slots for equipping Relics.
Dead Weight
Reduces the swimming speed and increases the sinking speed of the wearer.
Water Flows
Increases the damage dealt underwater
Water Attraction
Allows to use Riptide without rain or water (cooldown)
Location: Village Chest
Abilities
List of all the abilities of the Hunder Belt
Load Capacity
Increases the maximum number of slots for equipping Relics.
Training
Increases the damage inflicted by the wearer's pets
Location: Village Chest
Abilities
List of all the abilities of the Leather Belt
Load Capacity
Increases the maximum number of slots for equipping Relics