Warning: Contains spoilers for Fantastic Four #40
The entire Marvel Universe has a new enemy, the warmongering alien race known as the Reckoning, and in the latest issue of Fantastic Four it is revealed that they have such power to kill even the godlike Watchers with ease.
The Reckoning are one of the oldest species in the universe. When they were still young and underdeveloped, their planet Prosilicus was visited by the Luminous, known today as the Watchers, immensely powerful creatures who had reached such a high state of development and consciousness that they decided to travel the cosmos and share their "gifts" with others. The Prosilicans, however, misused the gifts of the Luminous, and turned them into instruments of war. They became ruthless galactic conquerors, forcing the other sentient species to form an alliance to fight them. In the final battle, a weapon of mass destruction of unspeakable power was released, turning nine-tenths of the universe into a toxic wasteland. The Luminous erected a barrier around what was left, sealing the Reckoning outside, and taking their famous oath of never interfering again with other species, thus becoming the Watchers.
The Reckoning hate the Watchers for abandoning them and are now back to exact their vengeance upon the rest of the universe. They possess Watcher-level technology, which makes them the most advanced race in terms of weaponry, but in Fantastic Four #40, by Dan Slott, Rachael Stott, and Jesus Aburtov, it is revealed that technology is not the only reason to fear them. Readers can see a glimpse of the true power of Wrath, the leader of the Reckoning, as he kills Qyre the Watcher with ease. After Wrath points out that a Watcher "outpowers the gods," he burns Qyre to the bones in a moment, proving that the Reckoning is even more powerful.
There is also a nice bit of retconning in this scene. The Watchers' "sin" that brought them to take an oath of non-interference was first recounted in 1964's Tales of Suspense #53. In that story it was revealed they gave the mysterious race (we now know it was the Prosilicans) the gift of atomic energy, as in the 1960s that was considered the most powerful and dangerous weapon. Now, Wrath calls it "cosmic fire" and in its display of power, it looks surprisingly akin to the Power Cosmic wielded by many powerful entities in the Marvel Universe, such as Galactus and his heralds. If every member of the Reckoning is able to wield that power, that makes them a threat unlike anyone that heroes have fought before, and that's without taking into account their plan to kill Eternity, thus ending reality itself.
Since their first appearance, it has been established that the Watchers are among the most powerful entities of the Marvel Universe, even if their pacifist attitude and their oath of non-interference mean that their power is often underestimated. If Wrath is capable of killing such the Watchers with such ease, the Fantastic Four and other heroes who will come face to face with him during the Reckoning War have no hope at all.