Skrulls

The Skrulls are a shapeshifting and technologically advanced alien race. They are the Kree's sworn enemies in centuries-long war, the Kree-Skrull War.

History
Over the course of history, they developed the ability to shapeshift into anything or anyone they want. They also fought the Kree for centuries.

The Skrull homeworld Skrullos was destroyed by Galactus. This was happened as a prophecy foretold it would. A second foretold that the Skrulls would claim Earth.

As the Kree sets their eyes on Earth, the Skrulls follow the same thing. They began infiltrating every human organization and groups on the planet, taking out and replacing each targeted member with no warning.

It is unknown if they can be detected by human technology or instincts, but it is proven that Doctor Doom's advanced technology can detect a Skrull in disguise as he found out that Invisible Woman was indeed a Skrull posing as the real one.

They also have very advanced alien technology and weaponry almost equal to the Kree in their war for centuries.

Power and Abilities
The Skrulls can shapeshift into any human or any superhero and supervillain as well in their infiltration on Earth to pave way for their invasion. They can also mimic their voice as well to prevent suspicion from other individuals.

Skrulls are shown to have enhanced speed, strength and durability, which is why Captain America was one of the easiest to replace since his powers are similar to them.

It is unknown how they duplicate a person's powers and abilities if a Skrull dosn't naturally have them (possibly using mutation or technology) such as Invisible Woman's shielding abilities and invisibility.

Weakness
Despite their morphing ability, they can still be seen using Earth and other alien technologies. Also if knocked out some or all Skrulls may revert fully or half way from their disguised forms. They can also slip out that they're imposters for reasons such as: actions, personality and memories (since a question or something else may be based on the person's past).

Notable Skrulls
Skrull Avengers: Super-Skrulls:
 * Captain America (Skrull)
 * Invisible Woman (Skrull)
 * Madame Hydra (Skrull) (Deceased)
 * Veranke (Mockingbird)
 * Henry Gyrich (Skrull)
 * Clay Quartermain (Skrull)
 * Lyle Getz (Skrull)
 * Black Panther (Skrull)
 * Captain America (Skrull) II
 * Hawkeye (Skrull)
 * Hulk (Skrull)
 * Iron Man (Skrull)
 * Thor (Skrull) (Deceased)
 * Wasp (Skrull)
 * Criti Noll
 * King Cobra (Skrull)
 * Rl'nnd
 * Super-Skrull