Essence

Essences are items that are used in the Trade Market. Currently, they can be obtained in two ways: through Monsters, for Attack and Damage essences, and through Quests, for Defense and Health essences. When traded at the Trade Market, they will upgrade your personal Runes in order to get bonuses to your stats for Guild battles and normal gameplay.

Essence Drops

At the moment, it appears that Essences will drop from monsters that have at least 300 million health. Additionally, monsters summoned through the normal Summon page will drop essences, while Festival monsters from either Tower of Transcendence will not drop essences. Monsters summoned in Guild Conquest do drop essences, though this functionality was not working properly at first.
For each monster that does drop essences, either Attack or Damage essences will be dropped (depending on the monster), and only one quantity will be dropped. It appears to drop after a fairly consistent amount of damage or stamina, so using smaller amounts of stamina will not increase your chances of getting more drops per stamina.
The above statement about stamina use is wrong.  Further research needs to be done to determine percentages of drop rate.
It seems that the game awards a quantity of essences after using 100 stamina on a monster. For example, one could do 2 50x power attacks and get 1 quantity of essence. This does, in fact, work with Generals that increase stamina usage.


Damage Essence can you get from

• Azriel
• Aurelius
• Jahanna
• Aurora
• Ambrosia
•  Azeron
• Fenix and many more 

   Attack Essence can you get from

• Alpha Mephistopheles
• Corvintheus
• Malekus
• Baal
• Alpha Bahamut
• Gehenna
• Glacius
• Typhonus
• Vermilion
• Kessaran        

Excavation Quests 

There are 4 new Excavation Quests available right now that will help you receive Defense and Health essences. They can be found in one of 4 quest areas: Land of Mist, Land of Water, Underworld, Mist II.
Energy used per level

• Level 1 Excavation: 300 Energy
• Level 2 Excavation: 400 Energy
• Level 3 Excavation: 500 Energy
• Level 4 Excavation: 600 Energy

Total needed for completion is 1800 Energy.

Land of Mist

• Level 1: Defense Essences x200
• Level 2: Defense Essences x500
• Level 3: Defense Essences x900
• Level 4: Defense Essence x1400

Land of Water

• Level 1: Health Essences x200
• Level 2: Health Essences x500
• Level 3: Health Essences x900
• Level 4: Health Essences x1400

Underworld

• Level 1: Defense Essence x205
• Level 2: Defense Essence x515
• Level 3: Defense Essence x930
• Level 4: Defense Essence x1450

Mist II

• Level 1: Health Essence x205
• Level 2: Health Essence x515
• Level 3: Health Essence x930
• Level 4: Health Essence x1450

 

 source :http://castleage.wikia.com/wiki/Essence

Trade Market

Players gain essences from attacks on some monsters, and as rewards from certain quests. The Trade Market allows players to trade these essences (Attack, Defense, Damage and Health essences) to participating guilds. The Trade Market is NOT an exchange for players to trade one type of essence for a different type. Players need not belong to a guild to trade essences, and need not trade exclusively with their own guild. Players who do belong to a guild will find their own guild always appears first in the trade market guild list. Beyond that, the guild list appears to be randomized.

Rune experience 

Each trade unit of 200 essences also costs the player 25 energy. In return, the player receives rune experience, which is applied to leveling up the player's corresponding personal runes (Attack Rune, Defense Rune, Damage Rune and Health Rune, as shown in the Keep). The player also receives 30 player experience. The base conversion rate is 1000 rune experience per 200 essences. A rune must be Forged onto equipment for the extra (Attack, Defense, Damage or Health) to activate. A trade bonus of experience (from 1% to 18%) can be applied, depending on your Conquest Duel Battlelust :

Battlelust	Trade Bonus
20 - 39	1%
40 - 59	2%
60 - 79	3%
80 - 99	4%
100 - 119	5%
120 - 139	6%
140 - 159	7%
160 - 179	8%
180 - 199	9%
200 - 219	10%
220 - 239	11%
240 - 259	12%
260 - 279	13%
280 - 299	14%
300 - 319	15%
320 - 339	16%
340 - 359	17%
360	18%

Essence storage 

To receive essence trades, guilds get one storage slot for each type of essence. Each storage slot's capacity starts at zero, and must be leveled up with guild-member contributions, in units of 5 iron, 5 wood and 25 energy. Players receive 30 experience for this contribution.
If an essence storage slot is full, no further trades of that type of essence can be received, until the guild expands that storage slot further. Storage level is limited by conquest guild rank. If a storage slot level is already equal to conquest guild rank, further attempts to contribute to storage expansion produce the message "You can not improve beyond your guild rank!".
Stored essence benefits the guild with guild-wide threshold bonuses.


Note : The 33% Guild Trade bonus effective in the beginning of the Trade Markey is currently ineffective, may be it is removed till further info.