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Summary
Black Goods come in a number of different Rarity Levels, ranging from "Extremely Common" up to "Legendary" rarity
The Rarity Level of a Black Goods item is randomized, with greater chances of Upgrading to higher levels based on the Creature Difficulty of the creature stolen from
Players who are Red Hand members of the Thieves Guild also get a 10% bonus to Upgrade chances for Black Goods they acquire from stealing
Players can Exchange Black Goods for random items via the Black Market in the Thieves Guild Menu
Any bonuses players receive towards "Special Loot Drop Rate" increases also increase the chance that Black Goods received from Pilfering/Mugging creatures will be of higher Rarity levels (this includes Special Loot bonuses coming from Weekly Server Region Bonuses, Guild Favors, Mastery Chain Links, Fortune Aspect Armor, and so on)