Warning: The following contains spoilers for Thor #17, on sale now from Marvel Comics.
Ever since confronting the Black Winter, Thor has been consumed by a nightmarish vision of the future and his own prophesied demise at the hands of Thanos. In this grim fate, the Mad Titan has completely razed the Marvel Universe and wields not only the powers of all six Infinity Stones but Thor's signature hammer Mjolnir. Commanding an army of the dead, Thanos easily crushes all the heroes who stand against him, including a defiant Thor. And as Thor begins to confide in more and more of his friends with what he has seen, he learns that this dark future may be one that not even his teammates on the Avengers can prevent.
Ever since Thor succeeded his father Odin as the new King of Asgard, he noticed that his father's enchantment on Mjolnir had grown increasingly unstable. While heavier for him to lift, others seemed to lift the magical hammer with relative ease, from his brother Loki to Iron Man. Realizing this made his visions of Thanos wielding Mjolnir all the more likely to come true, Thor began taking proactive measures to place the hammer under greater protection than he felt he was personally capable of, entrusting it for safekeeping with the Avengers at their headquarters in Avengers Mountain while he dealt with matters elsewhere. However, in Thor #17 (by Donny Cates, Michele Bandini, Matthew Wilson and VC's Joe Sabino), this decision appears to have backfired disastrously.
While called away to deal with an uncomfortable family reunion with his parents and half-sister Angela, Thor receives a phone call while talking with his mother Freyja. The caller is Captain America who has the unenviable task of relaying the news that Mjolnir has been stolen right from the heart of Avengers Mountain, with the superhero team unsure of who exactly took it and where the hammer and culprit have since gone. And with Thanos on the loose and menacing the Eternals and the Infinity Stones resurfacing around the Marvel Universe, it appears that Thor's prophecy has just taken another major step towards coming true.
Avengers Mountain itself has not been a particularly secure headquarters, as of late. Recently, the Russian superhero team the Winter Guard infiltrated the Avengers base of operations to kidnap She-Hulk right from under the noses of Earth's Mightiest Heroes, besting many of the Avengers before taking Jennifer Walters back with them to the Red Room. The Avengers have a long list of enemies, from the Squadron Supreme to the Defenders of the Deep, so Thanos being the true culprit behind the brazen heist isn't a guarantee, making this investigation all the more troubling and complicated.
The vision that Thor received from the Black Winter was not a one-time occurrence, with Thor privately revealing to Jane Foster that the vision appears to him whenever he dreams. And with everything occurring around the Marvel Universe, all signs are pointing to Thor's dark prophecy coming to pass, with a showdown against the Mad Titan, more powerful than ever, going poorly for the heroes.
Whenever Thanos holds the cosmic advantage, wielding all the Infinity Stones and any other upgrades at his disposal, the one truth in the Marvel Universe is that Thanos is inevitable.