This article contains spoilers for Death of Doctor Strange #1.
The greatest secret of the Sorcerer Supreme has finally been revealed in The Death of Doctor Strange #1. 1,000,000 years ago, the sorcerer Agamotto served as Earth's first Sorcerer Supreme. Since then, there has always been a powerful sorcerer sworn to protect this dimension from mystical threats. Doctor Strange is the latest to hold the title of Sorcerer Supreme, using the Eye of Agamotto to identify potential threats and wielding all his phenomenal magical power against them.
But Doctor Strange is secretly Marvel's darkest hero. To be Sorcerer Supreme is to always be fighting on the front lines, battling against villains ranging from Dormammu to Shuma-Gorath, confronting some of the most dangerous entities in all creation. Worse still, the title of Sorcerer Supreme is one that is coveted by every dark wizard, meaning Strange finds himself constantly assailed by mystics who would prove themselves against him. Suffice to say no Sorcerer Supreme will have ever seen the need for a retirement plan.
The Death of Doctor Strange #1 reveals the greatest secret of the Sorcerers Supreme - their true purpose. As he explains to the class of Strange Academy, the Sorcerer Supreme maintains the barrier, a mystic spell that strengthens the boundaries between this dimension and those surrounding it. "The Dark Dimension, the Celestial Concordance, the Consecution of Colors... all have attempted invasions of the Earth dimension in the past," Strange explains. "While travel among dimensions is possible, the barrier prevents an invasion en masse." The barrier appears to be bound to the life essence of the Sorcerer Supreme, as proven when it begins to collapse after Doctor Strange is murdered later in the issue.
The Boundary is clearly copied from the Marvel Cinematic Universe, where Doctor Strange revealed there is a mystical barrier protecting the Earth - one that is strengthened from three Sanctum Sanctorums, in London, Hong Kong, and New York. Kaecilius targeted the Sanctum Sanctorums in an attempt to destroy the barrier and allow the Dark Dimension to consume the Earth. It is quite possible that, in the comics, Doctor Strange has been killed for the same purpose; to allow a full-scale incursion on to Earth, with this entire plane of reality consumed by another realm.
It's worth noting that Doctor Strange never explained exactly how the Sanctum Sanctorum works in the MCU. It's entirely possible this barrier, too, is somehow bound to the Sorcerer Supreme, which would explain why Dormammu only invaded after the Ancient One had been killed. If that is the case, Earth is vulnerable right now - because Doctor Strange has not become Sorcerer Supreme yet in the films. The barrier may well have been degrading all these years, explaining why it is vulnerable in Doctor Strange in the Multiverse of Madness.
