This Hunt can be obtained after you have completed the events at the top of the Pharos of Ridorana including defeating Doctor Cid for the final time and after destroying the Sun-Cryst. Speak to Montblanc to obtain the Hunt.
The petitioner for this Hunt is Montblanc himself.
Shadowseer is located at the bottom of the Subterra of the Pharos at Ridorana. Check out the Ridorana page for more information about how to access the bottom, or ‘Unknown’ floor:
You should make sure to have 
White Masks and Black Masks in your inventory and hopefully you have one or two 
Ribbons available. Check out the section below for more information on obtaining them prior to the battle.
The 
Excalibur greatsword is also extremely effective during this fight. It is fairly difficult and time consuming to obtain, and this Hunt can be completed without it, but if you have not obtained it yet and would like to, check out the following section:
Once you have unlocked the ‘Unknown’ floor you can use the Dias of Ascendance to travel there directly. Be careful though as the battle against Shadowseer will begin as soon as the elevator stops.
Level: 67
HP: 278,078
Steal: 
Foul Flesh (55%), Maggoty Flesh (10%), Forbidden Flesh (3%)
Shadowseer is weak to Holy-type damage, so the Excalibur works extremely well during this fight. He will use Shining Ray at the beginning of the battle, making equipment that absorbs Holy-type damage quite effective. He will also use Fearga regularly which will drain your party members of MP. Be prepared to use 
Ethers, Hi-Ethers or 
Elixirs to replenish MP, or switch to items such as 
X-Potions to heal HP.
Once he drops to 80% HP, he will begin summoning some previous “marks” that you have defeated. He will summon Pandaemonium, then Slyt, then Fenrir and then Phoenix.
Pandaemonium is the most difficult of the four summons due to the length of time it takes to defeat it. Once it reaches 50% HP, it will cast Perfect Defense which will make it immune to attacks for a full 2 minutes. The three summons that follow (Slyt, Fenrir and Phoenix) are all straightforward and do not require specialized strategies to defeat.
Make sure to have your party members equip their Black Masks for the later portion of the fight as Shadowseer will switch to Dark-heavy Magicks. He will also cast a number of negative status effects, so Ribbon accessories are extremely useful for this fight.
A video of the battle in the Zodiac Age version is included below:
Return to Montblanc once Shadowseer has been defeated. The rewards for completing this Hunt include 20,000 gil and two Megalixirs.
