From Shaiya Wiki
Currently not yet added ingame due to some difficulties.
How to participate
1. Move to the battle zone waiting room located at the capital of either faction (Iris or Apulune). Players may do so until 30 minutes before the Goddess Battle starts. 2. 20 minutes before the beginning of battle, the gatekeeper will allow entry into the Goddess Battle Map. The gatekeeper is not removed until the Goddess Battle ends, so if you have left the map, you will be able to return if the battle is still going on.
Note: Ultimate Mode characters will not need resurrection runes when participating in the Goddess Battle!
The flow of battle
How to win
Defeat the opponent's goddess. If neither goddess is defeated before the event is over, the winner will be determined by the remaining hit points of each goddess.
To the victor go the spoils
Once the game ends, a reward gatekeeper is summoned. There is a separate gatekeeper for both Light and Fury camps. Your Goddess grants you passage.
Prize chests and monster summons are spawned by the reward NPCs.
Rewards are given to participants in both camps, but the quality of reward will depend on victory or defeat.
The battle ends
Players may move from the reward camp to the battle zone through the use of the /return command. The land is in order... until next battle!
Official page: http://shaiya.aeriagames.com/guide/goddessbattle