The Marvel universe is filled to the brim with heroes, and overflowing with villains for them to take a stand against. Around every corner is some sort of menace, and the next is always worse than the last. Currently, that particular ceiling is being pushed in some truly terrifying ways. One ancient, seemingly all-powerful evil is bad enough on its own, usually taking the collected efforts of heroes worldwide. For those who are defending the world today, a triple threat of otherworldly threats have reared their heads, and every single one of them is only a few short steps away from achieving complete and total domination.
Knull
The dark god of the symbiotes, creator of the living abyss, and slayer of gods is quite possibly Marvel's oldest known evil is finally here in King in Black. Coming from the abyss that existed before all of creation, Knull's reawakening has seen him raze and remake entire planets, aided by his unending horde of symbiote dragons. Now that he has arrived on our world, he is looking to do the same, and it doesn't seem there is anything that can be done to stop him. In just a few short moments, Knull's tendrils washed over the Earth's surface before enveloping the atmosphere as well, blacking out the skies and thrusting the world into darkness. Knull has faced off against the likes of the Silver Surfer, ripped the Sentry apart in brutal fashion, and torn the Venom symbiote from the only person who could resist Knull's call. Surely there is still some sliver of hope left, but whatever that may be has yet to reveal itself, and time is running out.
Mephisto
The Avengers have fought some incredible and memorable battles in their time. From Thor's earthshaking battle against Apocalypse hundreds of years ago to the Avengers facing off against Thanos in his pursuit of Death's hand, all the way to them having to beat down their own teammate, the Hulk, almost every threat that Earth's Mightiest Heroes have come up against is potentially world ending, and a recent revelation has shown that there was always a single set of hands pulling the strings. Mephisto, the Marvel universe's devil incarnate, has apparently been the mastermind behind some of the Avengers deadliest dust-ups and continues to do so. It isn't just their own universe's Mephisto that the Avengers are dealing with, but versions of the devil from alternate realities as well, all on top of the team's other ongoing struggles. It might seem like too much to handle, which at this point might even be just another web of discord that Mephisto has spun.
One-Below-All
The Hulk has been incredible for a long time, but only relatively recently has he taken up the title of immortal, though that is growing less true with every page that turns. Every time a gamma mutate dies, they go to the Below-Place, a world below even the deepest layers of Hell. There resides the One-Below-All, a malevolent entity who seeks only to destroy the rest of the multiverse, and is completely single-minded in its endeavors. The One-Below-All is also the source of the gamma energy that created the Hulk and his brethren, and what anchors them to the Below-Place in their deaths, as well as providing them the means of resurrection through the Green Door. The One-Below-All has found an acolyte in Samuel Sterns, aka the Leader, and he has discovered a way to control the Green Door, turning it red, and trapping any gamma mutates who find themselves in the Below-Place for as long as he wishes them to remain.