The connecting record
shownOverlord: The Witch of the Doomed Kingdom
Cocytus killed Brain Unglaus in Overlord: The Witch of the Doomed Kingdom.
a duel Brain asked for in the capital's streets; the body was frozen and kept
What each of them is ranked on
The killer · Cocytus · rank 5
Floor Guardian of the 5th Floor
Fought the lizardman chieftains and Zaryusu Shasha on the Great Lake by himself and killed each one who stood against him; Zaryusu, the last to fall, was revived by Ainz and returned swearing fealty. At the Kingdom's fall Brain Unglaus stepped into the road and asked him for a duel. Cocytus drove his halberd into the ground to draw a sword for it, and needed one stroke.
shownOverlord: The Lizardman Heroes · vol. 4, ch. 5
Fights the lizardman chieftains alone and kills them.
shownOverlord (anime) · S02E05
Shasryu and Zaryusu are the last two left facing him.
shownOverlord: The Witch of the Doomed Kingdom · vol. 14, ch. 4
Plants his halberd, draws the God Slaying Emperor Blade and kills Brain with a single slash.
All 4 citations
shownOverlord (anime) · S04E12
Exchanges names with Brain before their duel at the capital.
The victim · Brain Unglaus · rank 16
Lost so badly to Shalltear that he fled the Death Spreading Brigade's cave in tears. He came back with Nail Clipper, a combination of martial arts put together against her, and when they met again during the demons' night in the capital it cut through her fingernail. At the Kingdom's end he challenged Cocytus to a duel in the street and did not survive its single exchange. Contested against Gazef, whom he had spent years trying to surpass.
shownOverlord: The Bloody Valkyrie · vol. 3, ch. 2
Flees in tears after his strongest technique fails to injure Shalltear.
shownOverlord: The Men of the Kingdom Part II · vol. 6, ch. 11
Uses Nail Clipper on Shalltear and cuts through her fingernail.
shownOverlord: The Witch of the Doomed Kingdom · vol. 14, ch. 4
Blocks the Sorcerer Kingdom's march and challenges Cocytus to a duel.
All 4 citations
shownOverlord (anime) · S04E12
Bows to Cocytus in a warrior's respect before their duel.
What this does not settle
One death in one work. It is the only place the record puts these two in the same room, and it is evidence about that meeting and nothing wider — not a general matchup, not a rematch, and not a ranking of its own.
The ladder above it is argued from everything either of them has been shown doing; this page is argued from one citation.
Where the two disagree, the disagreement is the point, and it stays visible rather than being resolved by whichever of them happens to be on this page.