Doctor Strange in the Multiverse of Madness has just been released, and there are already questions to be answered. Surely one of them is the third eye that appears on Doctor Strange’s forehead in the ending scene of the film, while he is walking quietly through the streets of New York.
The answer can be found in the Doctor Strange comics, where sometimes the third eye appears on his forehead; this is nothing more than the projection of the eye of Agamotto that Doctor Strange wears as an amulet.
In the comics, the eye of Agamotto allows the wearer to recreate recent events and have powers of telepathy and to know/intuit the true nature of a person’s soul. When its power is used, a third eye appears on the wearer’s forehead (in the comics, the Eye of Agamotto can only be used by white magic wizards).
Therefore, it is likely that the evil version of Doctor Strange (Sinister Strange) was able to master this technique, which would explain his third eye on his forehead.