I do not own any of any of this, nor do i take any credit. To be safe, I added links in this so I don't look like I'm copywriting.

The Great Ones: (Belongs to Bloodborne)
The Great Ones are supernatural beings with incredible strength and stamina. They are demonic and godly in both appearance and abilities. The Celestial Emissary has the ability to disguise itself as a lesser Kin to deceive enemies, the Brain of Mensis can cause Frenzy with its stare, Mergo's Wet Nurse can create a nightmare realm, and the Moon Presence has the ability to levitate. They are basically both gods and demons in a way. They are a bigger threat than anything Dustin and his team have faced before. Their origin is unknown, but they have been around since the dawn of the Great Lords and the clash between the Great Lords and the Great Dark Lords. The mysterious nightmares are big, nasty, and down right ugly. It's up to Dustin and his friends to stop them. http://bloodborne.wikia.com/wiki/Great_Ones

Amygdala:

Amygdala is a monstrous entity with a spider-like body and what appears to be a fly-like head with many bulbous eyes on it. It appears to have hands on all but two of its limbs, which have feet. http://bloodborne.wikia.com/wiki/Amygdala

Brain of Mensis:

The Brain of Mensis is a massive brain-looking creature, covered in eyes. It is the reason why the Hunter takes frenzy damage after first arriving in the Nightmare of Mensis. A lever can be activated, enabling a cut scene showing the the Brain being lowered into the depths of the tower it overlooks. After this is done, the Hunter will not receive damage when walking outside the tower. http://bloodborne.wikia.com/wiki/Brain_of_Mensis

Ebrietas, Daughter of the Cosmos:

One of the eldritch Great Ones, Ebrietas is a monstrous creature that resembles a slug with squid like tentacles emerging from her back. Her face appears split in half and is covered in strange growths that resemble fungus that protect her vulnerable maw. Ebrietas possesses a set of skeletal wings that grant her limited flight.

The object of the Healing Church's worship, Ebreitas communes only with the highest members of the Church, guiding them in the ways of Blood Ministration. She is one of the Great Ones who was left behind in the old labyrinth, eventually discovered by the Healing Church. The Healing Church later founded the Choir to protect and study her. http://bloodborne.wikia.com/wiki/Ebrietas,_Daughter_of_the_Cosmos

Mergo's Wet Nurse:

Mergo's Wet Nurse is a hooded creature with an invisible head, six skeletal arms (each wielding a sword), and black tattered wings. She can create a nightmare realm with multiple copies of herself and is able to teleport.

Mergo's Wet Nurse is a Great One guarding the source of the nightmare. http://bloodborne.wikia.com/wiki/Mergo%27s_Wet_Nurse

Moon Presence:

An eldritch being composed of human flesh and bone, with the exception of its head.

The Great Ones spread the beast plague as a way to hasten the development of this creature, which is in fact a young Great One*. http://bloodborne.wikia.com/wiki/Moon_Presence

Rom, the Vacuous Spider:

Rom, the Vacuous Spider despite its title, resembles a gigantic pillbug rather than an arachnid. It has a silverfish-like tail and a bulbous body from which vegetation appears to be growing, possibly Coldblood Flowers. Its face resembles a chunk of pumice and is covered in eyes, enforcing its connection to the Great Ones. Rom is initially non-hostile until provoked at which point it will use powerful magic attacks to defend itself.

Rom is accompanied by creatures known as the Children of Rom. They resemble actual spiders but with the same shaped head and appear to have blades for legs. These spiders are possibly either the literal spawn of Rom or were created in order to defend the creature.

Rom, the Vacuous Spider controls the barrier that prevents normal Yharnamites from seeing the true horrors around them, such as the Lesser Amygdala's crawling around the Cathedral Ward. Rom was once a Byrgenwerth scholar whose human mind had been manipulated by the Great Ones, after asking Amygdala for "a million eyes on his brain." http://bloodborne.wikia.com/wiki/Rom,_the_Vacuous_Spider