Being a Seer with the eldritch Power to read the Fate of Men and Kingdoms by the Movements of the celestial Bodies and the Flow of Mist, twisted to a Thing of Evil by a great Flux of umbral Energy. The Shadowseer may draw on both the Power of Darkness and that of Light, being able to summon both Anima of Shadow and Anima of the Sun at once. Fearing its evil Will, Montblanc wisely recognized this Mark.
The Shadowseer is weak to Holy-elemental damage and absorbs Dark.
After losing a certain amount of HP, Shadowseer will summon Pandaemonium. During this time, Shadowseer will put up two palings, which essentially makes it invincible. Killing Pandaemonium will cause the palings to drop, allowing the party to injure it until it summons the Slyt. The Shadowseer will repeat this with the other Pharos bosses (Fenrir and Phoenix), at which point it will lament that it is out of creatures to summon.
The party should use Esuna whenever a member becomes inflicted with a status ailment. The party members most geared toward magick should cast Holy. The player may also want to equip Holy-element weapons, such as the Excalibur.
If the player is looking to deal a massive amount of damage, and willing to rob one character the use of magick, they could give a character (Basch is recommended) the Excalibur and Genji Glove, then cast Haste, Bravery, and Berserk, along with any other wanted buffs.
Shadowseer does, in a way, resemble a Zombie Mage as well as all the similar enemies of the undead genus. However, despite being vulnerable to Holy, Shadowseer itself is not an undead and is not recognized as one.
The four bosses that Shadowseer summons are references to the Sì Xiàng, four mythical Chinese beasts that correspond to the four directions of the compass. Pandaemonium refers to the Black Tortoise of the North, Slyt refers to the Azure Dragon of the East, Fenrir refers to the White Tiger of the West, and Phoenix refers to the Vermilion Bird of the South. It is possible that the Shadowseer himself is a reference to Heaven and Earth, which unite the four beings. The bestiary describes Shadowseer as determining the fates of those on earth, by the movements of celestial bodies, or the heavens.
Much like Doctor Cid and Gilgamesh, Shadowseer can be stolen from more than once. However, due to the permanent paling that activates when any of its summons are up, the only practical times to steal are at the beginning of the fight and after defeating Phoenix.