The Avengers are a fictional team in the mainstream prime reality of the Marvel Universe by Marvel Comics.
The Avengers are a team of superheroes whose membership has included humans, mutates, robots, aliens, and mutants. It's a team which has been around for decades, protecting the United States from both foreign and domestic threats, and the world from global, extra-dimensional, and alien threats. It has operated on Earth, the Moon, elsewhere in the Milky Way galaxy, neighboring galaxies (Shi'ar, Andromeda, etc), and other dimensions. Some of them have traveled in time or visited other realities. The team has battled aliens, demons, and gods. Some of them have been participants in universal scale events.
It's a team that has had a long history of conflict with Chthon an Old One. This being due to the history that the Scarlet Witch has with the demonic entity, which always end up involving the Avengers. Most recently, Chthon conquered the Earth, temporarily remaking it in his image, before what he did was reversed. Some time ago, they clashed with a supervillain (Grim Reaper) who was empowered by Lloigoroth, another Old One.