Warning! SPOILERS for Eternals
The end of Marvel's Black Panther: Wakanda Forever. The mutant son of a human sea captain and an Atlantean princess, Namor the Sub-Mariner is one of Marvel Comics' oldest characters - first appearing in Marvel Comics #1 back in 1939. Namor's focus on Atlantean affairs means he can shift between being both ally and antagonist to humanity's superheroes. He served alongside Captain America during the Second World War, and yet he's currently pursuing a war with the surface world in Jason Aaron's Avengers run.
The war between Wakanda and Atlantis.
Eternals seems to be another step towards Namor's appearance in Black Panther 2. The film opens with mysterious earthquakes affecting the entire planet Earth, causing a small amount of damage on the surface. Curiously, these earthquakes seemed to lack an epicenter, because they affected the entire planet. The Eternals later learned they were caused by the seed inside the Earth's core preparing for a Celestial Emergence. Later, the Eternals discovered a volcanic island in the ocean, apparently associated with a mantle plume that Tiamut was using to come to the surface. He tore through the oceanic plate, with one hand and his head emerging through the mantle, only to be transformed into stone by Sersi.
All this serves as potential setup for Namor the Sub-Mariner, who rules the underwater world. Atlantis could already have been disturbed by the earthquake in Avengers: Endgame that will have been unconnected to Tiamut, because it was before the Celestial's Emergence was accelerated by the infusion of life energy when the Hulk brought back half the living creatures on Earth. Further tremors associated with the Celestial may have only caused minor inconvenience on the surface world, but the same earthquakes could have caused real destruction in Atlantean cities. It's worth ing that, while Eternals' battle to prevent the Emergence is unlikely to have taken place near Atlantis, in the comics there are cities scattered across the globe that Namor is sworn to protect. Tiamut's Emergence could have caused untold havoc, with a massive loss of life. At the very least, the earthquakes associated with a gigantic being tearing through the crust would have been significant. Eternals could have been remarkably destructive for Namor and his kingdom.
Namor's people have presumably been keeping their heads down, attempting to prevent the surface world learning of their existence, but that will likely have become a politically controversial policy given the amount of pollution humanity is generating. Thanos' snap would have further destabilized Atlantis, and Eternals' subsequent earthquakes would have left the Atlanteans tired of being victims. That perfectly sets up the idea of Atlanteans emerging from beneath the waves in Black Panther 2, viewing the surface world with hostility.
