|Name|| Cuthbert Sinclair|
|Dates||1923 – 2013 (killed by Dean Winchester)|
|Occupation|| Men of Letters (formerly)|
Master of Spells
|Episode(s)|| 9.16 Blade Runners|
10.19 The Werther Project
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, and went on to design most of the warding that was used in the Bunker. He was also a mentor to Henry Winchester, teaching him how to use spells against his enemies. Eventually, 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 in May 16, 1956, Sinclair took the name "Magnus" and began collecting supernatural paraphernalia, 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 entrance 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 that makes anyone that attempts to open it, without doing 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; though he tells 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 not to be attempted to be opened again. However, when Abaddon destroys the Men of Letters, the plans to guard the Box are stopped and the chaperhouse is eventually sold off. Werther 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 under the influence of the Mark of Cain fully. 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' 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 blood disarm the enchantment, allowing them to open the Box and get the codex. Dean later destroys the Werther Box with a sledgehammer as a precaution.