San d'Oria reputation 4 needed.
Then head to Fei'Yin and kill
Underground Bats, Ore Golems or Shadows,
to get a Chestkey dropped.
This key is a really rare drop even with Treasure Hunter 2.
I killed like 200 Bats to get a Key.
After you got a Key dropped,
just find the chest,
get the Fei'Yin Magic Tome and
Head back to Quest starter NPC
(Eperdur - Northern San d'Oria)located at M-7.
I got my key from one of the ore golems. they drop a lot of mythril ore with TH2 so bring fire crystals if you have goldsmithing for a little extra farming on the side.
was just in fei' yin today. went to qu' bia zone and killed the 4 shadows to the east of it and got a 2 fei' yin chest key drops. i hope this helps maybe higher drop rate from them or just me being lucky.
They must have made the droprate better for this key was 65drk/whm and got two key drops in an hour or so. shadows have the best drop rate and weaker than the bats
As BLM73/WHM36 and the treasure hunter from the mog-tablets, i got my key on my 6th Shadow. was 11% moon at the time. It's not too hard, but much better drop rate than the other mobs.
I killed 6 bats with TH3 attained the key and got the chest i found the chest first that way i knew where to go and killed bats around that location and got the key item quickly and attainted the Scroll