Warning! Spoilers for Death of Doctor Strange: Avengers #1 ahead!
In Death of Doctor Strange: Avengers #1, it is revealed that the forces Doctor Strange will have to face in his last week are even more powerful than the King in Black. Before his death in Death of Doctor Strange #1, Strange conveniently explains to the students of Strange Academy that it is his responsibility as Sorcerer Supreme to maintain what he calls the Barrier. The Barrier is a master spell surrounding the Earth that keeps dark forces from the outer planes from entering our dimension. When Strange is killed by a mysterious enemy, the mystical Barrier falls and beings from other dimensions waste no time in invading.
Luckily for humanity, Doctor Strange used a spell to store a week of his life away, when he was much younger, to be used upon his death to sort out his affairs. The younger version of the hero appears in the Sanctum Sanctorum to the already gathered group of several mystical allies of Doctor Strange as well as Avengers Captain Marvel, Captain America, Thor, and Iron Man. Unfortunately for them, the considerable might of the assembled is nowhere near that of the new threat that lands at their doorstep, the Three Mothers. The unsettling new mystical characters quickly and easily defeat some of the mightiest Avengers ever assembled. To make matters worse, the Three Mothers are only the beginning.
In Death of Doctor Strange: Avengers #1 by Alex Paknadel and Ryan Bodenheim, a young girl from another dimension uses her superhuman abilities to build a spire to transmit a message to all of humanity. Her message is that she is the sole survivor of her dimension and the source of the devastation—known as the Child—is now making its way to Earth. The Three Mothers have determined the magic of the Earth will make a feast for the Child. When the Child arrives, it will be more powerful than any threat the Earth has faced before. This insinuates the Child will be even stronger than the overwhelmingly powerful King in Black, Knull.
In the King in Black event launched in 2020, Knull and his forces quickly overtake the entire planet. Knull tears Marvel's strongest superhero, the Sentry, in half. He then blankets the Earth in darkness with a shell of living abyss. He easily withstands every Hail Mary the heroes throw and proves himself to be nearly invincible. The only way he is defeated is by Eddie Brock hosting the "God of Light," the Enigma Force. Given what humanity just faced in King in Black, the Child being the greatest threat the Earth has ever faced is a bold claim.
The Child may live up to the claim, though, as he is said to be more powerful than the Three Mothers who have already proven to be devastating. The Three Mothers are named the Crown, the Wyrd, and the Crawling. All-Father Thor with all his might doesn't stand a change against the Crown, a warrior-queen armed with a massive sword. The Wyrd, a many-armed alien mage-priestess, beats Captain Marvel and Captain America seemingly without even turning her head or moving from where she floats. Worst of all is how the Crawling, an amorphous monstrosity comprised of acid-mouthed worms, defeats Iron Man. The worms enter Iron Man's suit leaving him helpless. He's left shaken like never before, having day terrors, no longer feeling safe in his armor, and struggling with his alcoholism.
In Death of Doctor Strange: Avengers #1, the henchmen of several mystical villains even team up to nuke a portal from which the Child could enter. Iron Man considers doing the same himself. Whatever the Child turns out to be, it will definitely be a formidable villain on par with the King in Black. Nothing less would be fitting for the final enemy of one of Marvel's most beloved and most powerful heroes, Doctor Strange.