Common- addition of stat on one attribute
Uncommon- addition of stats on one attribute
Rare- stats on two attribute
Superior - stats on three attribute + one skill
Epic percentage stats on 1 attribute+ one skill(active)
Legendary - percentage stats on two attribute+ one skill(active)
Heroic percentage stats on two attribute + two skills(active)
Heavenly- percentage stats on three attribute+ two skills(active)
Epic heavenly- percentage stats on three attribute+ two skills(active and passive)
Legendary heavenly - percentage stats on four attribute+ four skills(two active and two passive)
Immortal - no particular rule on it.
(I will be updating this from time to time, to add more information that I wouldn't want to add to the main story).