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I always thought that this referred to the rarity of the tiles that appeared, not a rarity bonus on the loot itself. Like, if you're farming black sun the unique helm node would appear more often.
This is correct. At higher corruption you're more likely to uncover echoes with rarer rewards such as exalted items or unique items. However the items from any individual reward type aren't affected by corruption, just level.
3 months later but just found this and the explanation was helpful. Does corruption also increase the chance of a rarer item to drop from echo bosses? For example, I've killed the boss of The Stolen Lance about 10-15 times but he ONLY has dropped one of the 2 set items each time. I need both of the other pieces he drops. is he more likely to drop the rare additional item with higher corruption?
Yes, each timeline boss has 4 specific drops. Two of these are common and it will always drop one or the other regardless of corruption. Then there are two rarer drops that have a drop chance that scales with corruption, and one of these two can only drop at 100+ corruption.
In the case of The Stolen Lance the set items are the common drops, Wings of Argentus is the rarer drop that requires 100+ corruption, and Throne of Ambition is the other rarer drop.