Super Chests, or sometimes also called special chests, are a special type of chest that have a significantly higher chance of dropping high runes than any other source in the game. They should not be confused with Sparkly Chests, because they have a completely different drop mechanic.
Super Chests can always be found in the same area and the same tile, but where (or if) the tile spawns in the area is randomized. This mean that they are much rarer than normal chests, but are much easier to find consistently.
A Super Chests does not use the usual Treasure Classes to determine its drops, nor does it use the normal quality algorithm to determine magic or rare drops.
Independent of which items are rolled, the quality is determined first. These chests have a 5% chance to drop all rare items and 95% chance to drop all magic items, this is unaffected by Magic Find. When the quality of the chest is rare, but an item cannot drop as a rare (like charms), it will just drop as a magic item.
Super Chest Item Quality
Each Super Chest is generated with a (kind of) 16 bit seed (2^16 - 2 = 65 534 possibilities). A chest will also have a chance to be locked or unlocked. The seed, and the fact that the chest is (un)locked will determine the items dropped from the chest. A combination of seed and locked state will always produce the same items, the affixes on the items are rolled separately.
This means that there are a total of 131.072 drop patterns, of which some contain high runes. Even if only one pattern had a high rune, they would still have a higher probability than Cows for example, given they first have to land in the Act 5 (H) Good on P7 (4.3%) TC, and then in Runes 17 (10.77%) TC, and finally roll a Vex rune or higher (0.13%). This gives us only a probability of 0.0006% per Cow drop.
Act 5 (H) Good
Luckily there are many more patterns that contain a high rune. We'll take a detailed look in the Lower Kurast Super Chest patterns below.
The Lower Kurast map always spawns a tile with 3(!) Super Chests. Sometimes, this tile can even spawn two times, giving you 6 Super Chests in a condensed area. Besides the Super Chests, there also plenty of armor and weapon racks, poppable objects like logs and corpses.
Since I don't know exactly how the chest seed translates into a drop pattern, I've relied on community efforts to compile these drop patterns.
Do you know how to calculate these patterns accurately? Let me know!
Super Chests in Lower Kurast have a 19% chance of being locked, which results in different patterns.
Given the locked probability, we can calculate the overall probability for P7/8. Here we only see a difference for mid runes, the amount of patterns containing high runes is the same for both locked and unlocked chests.
|unlocked (81%)||locked (19%)||weighted total|
This table show that overall, P7 or P8 is the optimal difficulty, having a total of 24 high rune patterns and 56.76 mid rune patterns. In the exceptional case you're only farming for a Lo rune, P4 or P5 would be marginally better.
Since you don't actually have to kill any monsters, there is no reason to farm this area on lower difficulty. Just make sure you have good survivability and high mobility.