Marvel Studios’ Doctor Strange in the Multiverse of Madness has recently surpassed $500 million at the worldwide box office.
In this article, I will talk about the Illuminati scenes that lead up to how Thanos was involved. This post does contain spoilers for a few scenes during the middle of the movie.
Earth-838

Doctor Strange and America Chavez appear on Earth-838 and see a statue heralding Supreme Strange as hero that died stopping Thanos, instead of Iron Man.
They are both swiftly captured by Karl Mordo, and Strange is then forced to appear on trial; infront of the Illuminati.
Illuminati Members

These members are: Mister Fantastic, Black Bolt and Captain Marvel (Maria Rambeau variant). As well as Captain Carter, Professor X and Karl Mordo.
The Illuminati blame all variants of Stephen Strange; for the desecration of reality. On Earth-838, they lied about Supreme Strange dying defeating Thanos. The statue they put up was also a lie to give ordinary people a hero to look up to.

Charles Xavier said: “Our Strange did not die defeating Thanos. We were at war! While the rest of us banded together to try to stop Thanos… Stephen as always chose to go it alone.”
Mordo interrupted: “He turned to the Darkhold, began dreamwalking, in hopes that our salvation may lie in the multiverse”. Furthermore, Mister Fantastic mentions, that Supreme Strange had caused an incursion; wiping out everyone in the universe, in another reality.
Death on Titan

Even in this universe; a Thanos variant was a major threat once again.
In order to stop Thanos, Strange had switched to using the Book of Vishanti, to take him down alone. The fights were not visually shown in the movie. The mad Titan can only be seen speared to death, on the right side of the scene; approximately 01:08:30 into the movie.
However, Supreme Strange was still too much of a threat, after sacrificing so many lives to stop Thanos. The Illuminati arrived and decided along with Strange; to wilfully agree to be destroyed by the voice of Black Bolt.
Read our review here for Doctor Strange in the Multiverse of Madness.