"Magnum Opus" is the tenth episode of Season Two of Fox's Sleepy Hollow. It was written by Donald Todd and directed by Doug Aarniokoski. It is the twenty-third episode of the series overall, and debuted on November 24, 2014.
In The Archives, Ichabod Crane and Abbie Mills play a guessing game to distract themselves from their research of Grace Dixon's journal, which contains a clue on a weapon that can kill Moloch. They later discover Katrina Crane using Henry Parrish's mirror to contact Ichabod, but their conversation is short. Henry later exploits the mirror to spy on the Witnesses instead, and learn of their plans to retrieve Methuselah's Sword. Henry suggests that Abraham Van Brunt use the remaining hours before daylight to try to retrieve the Sword before the Witnesses can. The quest is not without its perils, for the prophecy goes: "Know thyself completely or perish when you attempt to see."
As Ichabod and Abbie reach their destination, Abraham arrives on horseback. They hide from him as he attempts to enter the hidden lair. But he is distracted by Abbie accidentally knocking over a brick. The Horseman closes in on her hiding place, until Ichabod chooses to distract Abraham by yelling his name, so that Abraham will go after Ichabod instead. The sun rises, forcing the sizzling Horseman to leave. Ichabod recalls how Abraham, as his former friend, had shaped his destiny by encouraging him to come to the colonies.
The Witnesses go down into the hidden lair, and discovered many stone statues in the pit - people who have tried to retrieve the Sword, but were turned into stone by the Gorgon. Abbie Mills discovered that M. Dixon, one of her female ancestors, was one of these statues, causing her to feel a sense of impending doom. She and Ichabod decide to let the Headless Horseman battle the Gorgon, as the Horseman has no eyes to be his liability.
Night falls, and Abraham returns to continue his mission. Ichabod used the camera on his smartphone to monitor the fight between the Headless Horseman and the Gorgon, while carefully avoiding direct eye contact with the Gorgon. In this tense moment, Abbie wondered if Ichabod was "trying to take a selfie with the Gorgon".
While the fight goes on, the Witnesses enter the hidden chamber, which contains many swords surrounding a large container of liquid. Ichabod warns Abbie of the possibility that seekers may have only one chance to pick out the correct sword, before he heads out to continue monitoring the battle.
Abraham wins the battle by beheading the Gorgon. After that, his head became visible to Ichabod (and later, Abbie). The two former friends engage in a duel, while Abbie chooses a sword. All the swords on display, including the one Abbie picked, turned into snakes, so she tells Ichabod (and later, Abraham) that there is no sword. Abraham enters the chamber, and put a gun at Ichabod's chest, declaring that his choice of his identity as the Horseman of Death. In response to the shofar sound, Abraham leaves the chamber to join his master, Moloch.
Ichabod and Abbie discover that the container of liquid contains oil instead of water. A single torch lowered into it causes the flame to be extinguished. But when both Witnesses put in their torches together, Methuselah's Sword is revealed.
- In Japan, this episode is known as The Invincible Sword (無敵の剣 Muteki no Ken).