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The Arena Champions, The Mountain Beneath, and Cremorus do not guarantee unique drops. Of dungeon / arena bosses, only Julra guarantees a unique. The reason for this is that the dungeon reward for Temporal Sanctum requires a Unique item with Legendary potential, and an Exalted item of matching base type to utilize the reward mechanic. We wanted to make sure with these systems that you can always utilize the reward mechanic.
With this, the Temporal Sanctum boss (Julra) will always drop an LP unique and an exalted item matching the base type. The first three modifiers for the Vaults of Uncertain fate are discounted so they just gold drops gotten through the dungeon itself the vault can be opened with some contents, and the Soul Gambler is completely self contained within Soulfire Bastion.
Arena Champions do not have a “reward mechanic” like dungeons, so there’s no specific demand on providing a specific item with the drop to ensure something further isn’t completely locked out that a player should have access to. Arena keys are also far more common than Dungeon keys as they can be specifically targeted as monolith rewards. With this, the “rewards” for completing the Arena are through the modifiers along the way, and the tier modifier.
If there was always a guaranteed drop, and as they only have a single linked Unique Item, the power level of the Uniques for them would have to be kept lower as there is no longer any rarity factor - Primarily this would make it far too easy to get much higher rolls on the item. This may potentially change down the road as we continue to add more variety, but we made the decision not to make them guaranteed so that we could provide more power to the uniques and provide more reason to revisit them after the first encounter.