Depending on the difficulty slider setting, every time an Act Boss is killed on Hell difficulty, there's a chance for the boss to drop a piece of their respective set. If the chance occurs, the specific item from the set is picked at random, with equal probability for each item in the set.
Mini-bosses are bosses spawned by activating a Boss Shrine, or on the last two floors of Chaos Tower -- one on the 9th floor, two on the 10th floor (Hell difficulty only).
Uber Damien has a 10% chance to drop an item from any Act Boss set, excluding Odin. If the chance occurs, an item is picked at random from among all of the sets, with equal probability.
Uber Reaper is found in the Shadow Realm. Shadow Realms are random spawned zones throughout maps on Hell Difficulty. You will need 10 Dimensional Shards to enter the Reaper's Breach.
Uber Reaper has a 20% chance to drop an item from his loot table (not including Ruby Key). If the chance occurs, an item is picked at random from the four items, with equal probability.
War God Sung Lee has a 20% chance to drop an item from his loot table. If the chance occurs, an item is picked at random from the five items, with equal probability.
King Rakhul has a 20% chance to drop an item from his loot table. If the chance occurs, an item is picked at random from the five items, with equal probability.
Grimbone is found in the Niflhel dungeon in Helheim, which requires a Helflame Torch to enter.
Grimbone has a 10% chance to drop an item from any Act Boss set, excluding Odin. If the chance occurs, an item is picked at random from among all of the sets, with equal probability.
Sheep King is found in Sheeponia. To open the portal, bring a cloud key to the dragon head statue with the wiggling tentacles in the town of Inoya.
Sheep King has a 10% chance to drop an item from his loot table. If the chance occurs, an item is picked with the following distribution