Essence gear begins to show up in the later levels towards Gondor. In short it is a way to enhance you gear through slottable "essenece" . Essense come in all kinds of flavors. The start around a rank 5 and go up to rank 12 I believe but I could be wrong.
An Essence is essentially a way to hone in your character's your role. So as tank I could have piece of gear with 2 essence slots, they could be a tier 9 with +4k block and maybe +3k tactical mitigation.
Gear can have up to 4 essence slots
Essences can be crafted as well and I believe that is when the higher ranks will come in.
The thing to keep in mind is when stacking, you will have a hard cap. So a hard cap on Critical Defense is 80%, so you can't have more then an overall score of 80% on that cap. SO each class will have it's own hard cap for every attribute from vitality, might , to Tactical Mastery ect.
Essence is one of better enhance modules of seen in mmorpg's in terms of usability and through output
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