FFXIclopedia
FFXIclopedia

needed to open the silver door in sea serpent grotto; trade to the door; you keep the silver beastcoin after door opens; opening door for the treasure chest to get the map of the area. --LDf5% 00:32, February 15, 2010 (UTC)

This can also be stolen from various Sahagins in Sea Serpent Grotto.