Rom the Vacuous Spider appears to be a Great One and Kin from Bloodborne. It acts as the barrier that deters normal Yharnamites from seeing the eldritch horrors that surround them, such as the invisible Lesser Amygdalas roaming around, hanging off of the Yharnam skyline.
Several theories suggest that Rom was once a scholar of Byrgenwerth that, through the courtesy of Kos (another Great One), went through a rapid period of evolution and gained incredible insight (i.e., "eyes").
Presumably, Rom is that which binds Provost Willem to the Moonside Lake- Rom has transcended humanity and has become a god like figure by gaining eyes on the inside. Willem now seeks to do the same.
Powers and Stats
Tier: At least 5-C, possibly High 4-C
Name: Rom, The Byrgenwerth Spider, "Retard"/"Idiot" (Referred to this by the Tomb Prospectors in the original Japanese)
Age: Unknown, but apparently very old (Presumably originates from the time of Willem and Laurence- by the time the Hunter arrives, Laurence is naught but a decaying Cleric Beast skull in the Grand Cathedral, and Willem is long dead)
Classification: Great One
Powers and Abilities: Superhuman Physical Characteristics, Magic, Summoning, Teleportation, Immortality (Type 1), Curse Manipulation, Ice Manipulation, Water Manipulation, Possibly Pocket Reality Manipulation to a minor extent, Portal Creation, Mind Manipulation, Illusion Creation, Reality Warping
Attack Potency: At least Moon level (Capable of holding the Paleblood Moon in place), possibly Large Star level (Equal to the Hunter and superior to Micolash, both of whom are well capable of using A Call Beyond, which is literally a stellar explosion; note that Call Beyond is not yet found by The Hunter and it is only an assumption that Rom is superior to Micolash)
Lifting Strength: At least Class P (Capable of holding the Moon in place)
Striking Strength: At least Class XTJ
Durability: Possibly Moon level (Able to withstand attacks from the Hunter, whom she is roughly comparable to at this point in the game)
Stamina: Relatively low- typically takes a few actions and then rests momentarily
Standard Equipment: None
Intelligence: Super Genius, all Great Ones are on a level of thought vastly beyond what humans can achieve while remaining human, and are all together considered to be on an entirely different plane of thought
Weaknesses: Minor weaknesses to fire, electricity, and magic