"Blinded, maimed, buried and surrounded by the fleshy mound created by the soiled bodies of the Marrow cancer. His eyes have been taken to the fringes, guarded by those dear sweet Emissaries. That shameful crime I helped produce is caught in the deepest place it could be brought, now haunted over by those few selfless minds left behind. This should be enough, but it won't be. Instead I must bury all of it, tower the earth above this valley and envelop all beneath a mountain of dirt. Will it be enough? Men are prone to dig. My hope that the terror of Frederic's wake will be enough to ward off any curiosity, before any travel further below. Curiosity has ruled me, and now may have doomed us all, if my measures are not enough to silence these beacons."
The Imprisoned Seer is a an optional boss at the end of an optional section of Marrow and possess the final weapon (Tendril), a very powerful weapon which is needed in order to get the game's true ending.
The road to the Seer requires a careful eye and a good memory of your previous surroundings. It isn't enough to simply reach his cabin, the entrance to the Seer's prison isn't so easily broken in to. In order to open the path to the Seer the player must collect a number of the Seer's eyes, hidden in secret rooms around the game world, and protected by a set of mini-bosses. Finding these locations is where your memory and instincts have to be sharp.
The boss hangs from the ceiling and fires projectiles at the player. Each projectile spawns with a random colour. Use the orb to destroy the ones which spawn from his hands and reflect those which come from his head, that is the only way to damage the boss. Striking the boss with your weapon will damage you so be cautious about engaging the other enemies in the room during the fight. Two of the "Taken" enemies will continuously spawn during the fight, in addition to the enemies which spawn from the Seer's dripping blood. Managing the other enemies and timing your attacks are the keys to winning this fight, which can be a bit of an endurance match given how difficult he is to damage.
In Echo mode, this boss fight plays out a bit differently. Since you can't distinguish colours in Echo mode the Seer's projectiles will always spawn in a state that can be reflected only with a weapon strike. It seems like a small change but the challenge is ramped up significantly due to the position of the attack and the precision needed to hit it. Interestingly enough, due to this change the boss doesn't technically require the Luminous Orb to defeat.