Legendary items are copies of "regular" items but come with a minimum of HP, Mana, Endurance, depending on their Tier (e.g. 140 for Tier 6) and an additional augmentation slot of type 6 for weapons or type 15 for other items. Legendary weapons often have improved proc rates (150 instead of 100%). Legendary items with a click-effect sometimes have the cast-time of the effect reduced.
Boss NPCs can have none or up to three different chests (Legendary, Mighty and Mythical/Ancestral). If a treasure chest appears, and which one it is, depends on the difficulty mode, the rarity of the chest and bit of luck.
Many chests have a rarity of "common", which means if the NPC is defeated in the difficulty mode "Normal" or above, a Legendary Treasure chest appears. If defeated in a lower mode, the chest may or may not appear.
When an NPC has access to multiple treasure chest types, there's a chance for the superior variant, the Mighty Treasure Chest, to spawn. This likelihood increases as the difficulty mode rises. Typically, a reasonable chance begins at Hard-, while Intense mode offers a significantly elevated probability of encountering it.
Rarities:
Rare: a chest (such as a legendary treasure chest) is guarnteed if the instance is in difficulty mode Intense; in lower modes it's more likely to not obtain a chest
Uncommon: a chest is guaranteed in Hard- and above.
Common: a chest is guaranteed at Normal and above.
Tier+: a chest is always appear from Normal- upwards; this rarity is called tier+, because it's often applied to NPCs who are one tier above the instance's tier (e.g. a tier 11 NPC in an instance that's classified as tier 10).
Tier++ chests are guaranteed from Easy+ and are usually tied to NPCs two tiers above.
Tier+++ chests are guaranteed from Easy and are usually tied to NPCs three tiers above.
The table below shows all NPCs who can spawn legendary treasure chests.
Example:
The Fabled Lord Nagafen common (2)
This means:
common: If defeated in Normal or above, a Legendary treasure chest is guaranteed
(2) This NPC has a total of 2 possible treasure chests. The chances for the Mighty Treasure chest are slim in Normal and get higher with every increase of difficulty mode. They are very high on Intense.