Doctor Strange In The Multiverse Of Madness: 10 Biggest Questions It Leaves Unanswered
4. How Exactly Did Earth-838's Heroes Kill Thanos?
Mid-way through the film, Earth-838's Illuminati reveal that, with the help of their Doctor Strange, they managed to defeat Thanos in the past, though the particulars of that encounter are sadly never shown.
We do however see a brief glimpse of the fight's aftermath, with Thanos impaled by what appears to be his own giant sword, yet it isn't conclusive that this is what killed a man who famously implored Thor to "go for the head."
Professor X (Patrick Stewart) mentions that the Illuminati were able to defeat Thanos with the help of the Book of Vishanti, which in both the comics and movies can be used to perform powerful magical counter-spells.
Perhaps the Book of Vishanti was used to negate Thanos' use of the Infinity Stones or allowed them to make Thanos vulnerable in a way that his own sword could be used to mortally wound him.
It's a little ambiguous and a damn shame, too, because it would've been fantastic to see an alternate version of Thanos' last stand on Titan.