Mother Brain?

The Mensis nightmare has 3 great ones in it. the Wet Nurse, Mergo, and Mother Brain.

What is speculated is that Mergo is the child of Queen Yharnam, who was found alive in the chalice dungeons, along with the Cord of the Eye.

According to its description, this cord granted the School of Mensis audience with the stillborn Mergo. This is similar to how the Choir used a chalice to be granted audience with Ebrietas. Mergo is speculated to be a child of Oeden, and the Wet Nurse a servant of Oeden. The connection between Formless Oeden and the Wet Nurse helps explain why the Wet Nurse seems to have no form, they are related great ones.

What the School of Mensis did with Mergo was use him as bait. Basically, all the Great Ones would hear his cries and follow it, because they themselves seek children.

The Brain of Mensis may have arrived in the nightmare in a few ways that I can think of.

My first idea is that the brain was dumb enough to take the bait and it got captured by the School of Mensis, which discovered it's eyes were evil and its brain was rotten (source: Living String). Finding no valuable use for it, they decide to just chain it up and use it as a frenzy cannon for their lair.

My second idea is Mergo's family gave the School of Mensis a crappy present in exchange for taking care of Mergo. You'll notice a large number of corpses near where Mergo's crib was. These can either be sacrifices, people whose died just looking at Mergo, or those killed by Mergo or the Wet Nurse for getting too close.

My final idea is that the brain was there when the School of Mensis first landed in the nightmare. They might have fought with it a bit before it was captured and chained it up.

/r/bloodborne Thread