Tiny's Trial is a crafting station minigame found in Betrayal league safehouses when Guff is one of the members of that division.
Players have a limited time to apply various currencies to one of their items. The currencies are somewhat random but include high-level items such as 
Chaos OrbsChaos OrbStack Size: 10Reforges a rare item with new random modifiersRight click this item then left click a rare item to apply it.
Shift click to unstack.
and 
Exalted OrbsExalted OrbStack Size: 10Augments a rare item with a new random modifierRight click this item then left click a rare item to apply it. Rare items can have up to six random modifiers.
. The currencies are provided by the game and are not taken from the player's inventory or stash.
The experience can be simulated at https://poesimulator.github.io/trial/. In this simulation, players left-click to apply currencies, rather than picking items up with the left mouse button and applying them with the right, as they do in the real Tiny's Trial.