Rom, the Vacuous Spider: the Great One Under Byrgenwerth's Lake

Rom, the Vacuous Spider, is a Great One hidden at the bottom of the lake at Byrgenwerth, and the boss whose death turns Bloodborne's moon red. A note in the college says "the spider hides all manner of rituals, certain to reveal nothing, for true enlightenment need not be shared." The trophy for killing it reads "Defeat Great One: Rom, the Vacuous Spider." (Bloodborne)

What the game says about Rom#

Rom is named in two notes before the hunter ever meets it. One, left in the Cathedral Ward, reads: "The Byrgenwerth spider hides all manner of rituals, and keeps our lost master from us. A terrible shame. It makes my head shudder uncontrollably." The other, inside Byrgenwerth, is the line about rituals and enlightenment that need not be shared. The "lost master" is usually taken to mean Willem, who never leaves the college. (Bloodborne)

The Lunarium Key points the way. It says Willem, in his final years, was fond of the lookout over the lake, and that "in the end, it is said, he left his secret with the lake." Willem sits on that balcony in his rocking chair. Jumping from it into the water drops the hunter into Moonside Lake, where Rom waits. (Bloodborne)

The Lake rune, made by the Byrgenwerth runesmith Caryll, is often read alongside this. It says "great volumes of water serve as a bulwark guarding sleep, and an augur of the eldritch Truth. Overcome this hindrance, and seek what is yours." (Bloodborne)

The fight in Moonside Lake#

Rom doesn't start the fight. It lies still in the shallow water, surrounded by smaller creatures that look like spiders with the same armoured heads. Only once attacked does Rom fight back, raining crystals down from the sky, raising them out of the water and teleporting away across the lake when hurt. It drops Kin Coldblood, described as coldblood "of inhuman kin of the cosmos, brethren of the Great Ones". (Bloodborne)

Rom isn't really a spider. It has a long pale body, many legs and a lumpy head crowded with eyes. The English title calls it vacuous; the Japanese name uses the word for "idiot". The game never explains either. (Bloodborne)

Why the moon turns red#

Killing Rom changes the night. A ghostly woman in a bloodied dress appears by the water, looking up at a moon that has turned red and huge. Most players identify her as Queen Yharnam of the Pthumerians, who stands in the same pose in the Nightmare of Mensis, though the scene never names her. (Bloodborne)

From then on, Yharnam is different, and new notes appear. A note in the Yahar'gul chapel reads "Ritual secret broken. Seek the nightmare newborn", which points toward Mergo. Another warns that "madmen toil surreptitiously in rituals to beckon the moon." The way to the Nightmare of Mensis and the School of Mensis opens. See the story explained for where this falls in the plot. (Bloodborne)

The usual reading is that Rom's ritual hid the truth from human eyes, and that the red moon was there all along. A Yharnam note puts it in a single line: "When the red moon hangs low, the line between man and beast is blurred." The game never spells out exactly what Rom's ritual did. (Bloodborne)

Rom, Kos and the eyes#

The only clue to Rom's past comes from Micolash, Host of the Nightmare. Mid-chase he prays: "Ah, Kos... Or some say, Kosm... Do you hear our prayers? As you once did for the Vacuous Rom... Grant us eyes, Grant us eyes!" Kos is the dead Great One of The Old Hunters, and eyes are what Byrgenwerth and Mensis chased. (Bloodborne)

The common reading follows from that line. Rom was once a person, probably a scholar of Byrgenwerth, and Kos granted Rom eyes, turning Rom into a Great One. The note's phrase "the Byrgenwerth spider" fits that. But nothing names Rom as a student, says when it happened, or confirms Rom was human. (Bloodborne)

What the game never confirms#

Who set Rom in the lake is the biggest open question. Willem "left his secret with the lake", and many players think he placed Rom there himself to hide the truth from the Church. Others think Rom chose the lake. The text only puts the two side by side. (Bloodborne)

The second question is whether there were more of them. A creature with Rom's shape lies dead at the Altar of Despair, where Ebrietas crouches over it, and Rom can be fought again in the Chalice Dungeons. No item says whether these are other spiders or other versions of Rom. (Bloodborne)