There are three different reward rooms and it is unknown if it works randomly or if it is triggered by a special condition, but the three rewards are:
Left room: Most of the team will be sent to this room. The rewards are: a Bag with 5 Platinum Coins, 1 Gnomish Supply Package, 1 Rusted Armor and 1 Piggy Bank.
Center room: Three players will be sent here. The rewards are: a Bag with 50 Platinum Coins, 2 Gnomish Supply Package, 1 Slightly Rusted Armor and 1 Piggy Bank.
Right room: One player will be sent here. The rewards are: a Bag with 2 Crystal Coins, 4 Gnomish Supply Package, 1 Slightly Rusted Armor, 1 Slightly Rusted Legs and 1 Piggy Bank.
Source:
https://tibia.fandom.com/wiki/Dangerous_Depths_Quest