|Name|| Cuthbert Sinclair|
|Dates||1924 – 2014 (killed by Dean Winchester)|
|Occupation|| Men of Letters (formerly)|
Master of Spells
|Episode(s)|| 9.16 Blade Runners|
10.19 The Werther Project
15.14 Last Holiday
Things never change do they? I kept telling the boys over and over again, I would say, 'We can stop all this, we could rid the world of monsters once and for all, if we just put our minds to it.' But 'Oh no,' they said, 'no, no, no, it's not our place, we're here to study, we're here to catalog.'
– Magnus, 9.16 Blade Runners
Cuthbert Sinclair was a former member of the Men of Letters. He was named "Master of Spells" soon after his initiation, which took place in the 1930's before World War II, and went on to design most of the warding used in the Bunker. He was also a mentor to Henry Winchester, teaching him how to use spells against his enemies. The projects he proposed for the Men of Letters in his last few years as a part of the organization were rejected by the leadership for being eccentric and irresponsible. This eventually lead to his complete separation from the Men of Letters in April 1956 after an invention of his, the Werther Box, led to the death of two fellow members. After the formal separation on May 16, 1956, Sinclair took the name "Magnus" and began collecting supernatural paraphernalia and spells, as well keeping a "zoo" of monsters in a magically-hidden fortress in the middle of the woods.
Sinclair mentioned to the Winchesters that he was always fond of Henry and that Henry even kept visiting him in secret the same way the Winchesters did before he traveled through time in 1958. Sinclair used spells to hide his fortress, disable his enemies, and even maintain his youth. He also used certain monsters from his zoo as a security measure should anyone invade his fortress.
With the help of Crowley, Sam and Dean are able to locate Sinclair's fortress. Once there they reveal themselves as Henry Winchester's grandsons and Men of Letters. Magnus then creates a door for them to enter and goes about testing them against two vampires from his zoo to see "what they are made of." Once the Winchesters kill the vampires, Magnus sits down with them to hear them out. They relay to him the threat of Abaddon and that they need the First Blade to take her down. Upon seeing the Mark of Cain on Dean's arm, Magnus uses a spell to send Sam back to the outside, while he tries to sell Dean on becoming a part of his collection and living forever.
When Dean refuses, Magnus takes to using magic to get Dean to be compliant and places the First Blade in his hand to see what would happen. When Sam and Crowley find a spell that will gain them entrance to fortress, Magnus attempts to use Sam against Dean to make Dean see things his way. While Magnus is torturing Sam, Crowley is able to release Dean from his chains, allowing him to grab the First Blade and decapitate Magnus.
While looking for information on a codex that could decode the Book of the Damned, Sam finds out that Magnus hid it in May 1956 inside the Werther Box, a magical vault of his own creation that causes hallucinations which make anyone who attempts to open it without the proper ritual kill themselves. Sam listens to the tape of the meeting where Magnus was expelled, in which he defends his actions in creating the Box because he felt the codex was too dangerous to allow the Grand Coven to recover. However, the other Men of Letters were not impressed, as the Box caused two of their members to kill themselves with its alarm system. Sinclair tells the other members that they should be out fighting evil with the knowledge granted to them, not sitting around in the Bunker like librarians. Markham gives Sinclair a chance at leniency if he tells them how to break the warding spell on the Box, but he refuses and is escorted out. He does tell them that Martinez, a Man of Letters who had died trying to disarm the Box, was on the right track, though he doubts the Men of Letters have the courage to try. Rather than try, like Sinclair expected, Markham orders the Werther Box to be interred and guarded where it stands, in the house of the St. Louis chapter, and that no further attempts be made to open it. However, when Abaddon destroys the Men of Letters, the plans to guard the Box are stopped and the chapterhouse is eventually sold off. The Box would remain untouched in the basement of the house until in the 1970's, a young girl named Suzie attempts to open it, which causes her entire family to kill themselves.
In the present, Sam and Dean attempt to disarm the Box. Sam's attempt at using a spell fails as he, Dean, and an older Suzie fall under the Box's influence. Suzie kills herself and Dean falls into a catatonic state where he imagines he is in Purgatory, where Benny Lafitte encourages him to kill himself so he can stop Sam and Castiel from having to do it when he falls fully under the influence of the Mark of Cain. Sam is apparently saved from a hallucination of Suzie by Rowena, but it is just another of the Box's hallucinations. Sam eventually discovers that Magnus designed the Box so that only the blood of a Man of Letters or a Legacy could break the spell, but that it needs enough to take a life. Sam attempts to sacrifice himself to stop Magnus's creation and save Dean, but Dean breaks free of the spell and stops Sam, adding his own blood to the mix. Sam and Dean's combined blood disarms the enchantment, allowing them to open the Box and get the codex. Dean later destroys the Werther Box with a sledgehammer as a precaution.
Cuthbert appears in a pre-war archival film found by Jack where he demonstrates the power that a wood nymph is capable of before any Men of Letters watching the film. According to him, wood nymphs are naturally docile, but they react violently when home and family are threatened. Sinclair has Mrs. Butters remove the head of a Thule agent with her bare hands after he is deemed to be no longer needed. As a result, Mrs. Butters joins the Men of Letters for service and security as the Bunker's housekeeper.
Mrs. Butters reveals that Cuthbert tortured and indoctrinated her into a black and white view of the world, causing Mrs. Butters to try to kill Jack. When the Winchesters refuse to kill him, she locks Dean in the dungeon with Jack and started torturing Sam in an effort to persuade him to see things her way. Sam and Dean convince Mrs. Butters that Jack isn't a threat and that Cuthbert had only been using her, telling her that Jack can fulfill the Men of Letters' primary mission: save the world from all the monsters of the world. They realize that despite being convinced to help Sinclair and the Men of Letters, she ultimately wanted to return home. Before Mrs. Butters leaves, the Winchesters and Jack give her a picture of the Men of Letters, which included Cuthbert.