Marvel Comics is well known for its earth-based and relatable heroes and villains. But there are many bizarre and unusual alien species in the Marvel Universe who have played prominent roles in some of the publisher's most important storylines. Occasionally, these extra-terrestrial beings are presented as allies and teammates to the world's champions.
Often though, they are portrayed as dangerous antagonists with malicious motivations. In addition to the threats posed by their super-powered human villains, there are many dangerous alien races Marvel's heroes have been forced to contend with.
Dire Wraiths
An ancient offshoot of the notorious Skrulls, the Dire Wraiths retain the same shape-shifting abilities of their progenitors but have mastered their own form of dark sorcery in the thousands of years since their separation from their parent species.
Continually seeking to expand their dark empire, the Dire Wraiths set their sights on the planet of Galador, leading to a decades-long war. To combat this relentless invasion, the Galadorians permanently physically modified a select group of their greatest warriors, creating the cybernetically enhanced Space Knights.
Chitauri
One of the more recent additions to Marvel's cosmic pantheon, the Chitauri are another shape-shifting alien race who were first introduced to the Marvel Cinematic Universe in 2012's film, The Avengers, and led by Loki, but have since made their way into the comics. A species of savage, simple-minded creatures, the Chitauri possesses limited intelligence, save for their Queen, whose orders they follow unquestionably.
In contrast to the Skrulls and Dire Wraiths, the Chitauri lack the ability to change form without first consuming a host, whose appearance they are then able to assume. Doggedly persistent, the Chitauri possess no sense of individuality and instead operate as a relentless force of "drones" commanded by the will of their Queen.
Klyntar
Better knows as "symbiotes," the Klyntar are an amorphous, parasitic alien species who survive by bonding with hosts and merging with them in both body and mind. The most well-known representative of this species is the Venom Symbiote, which was brought to earth by Spider-Man after the interstellar Secret Wars event.
Since being introduced to the planet, the Klyntar have replicated, creating new, earth-born symbiotes, as well as paving the way for others of their species attempting invasion. With the ability to carry physical traits learned from one host to the next, such as replicating Spider-Man's webbing in Venom, the Klyntar are perpetually evolving and upgrading themselves as they utilize these gained abilities to entice future willing hosts in their search for a permanent union.
Shi'ar
The Shi'ar species itself is an avian humanoid race retaining many bird-like attributes, such as feathers in lieu of hair, and wings in some members possessing the necessary recessive genes. While not necessarily a physically imposing presence alone, the danger of the Shi'ar lies in their massively expansive empire, encompassing multiple worlds and their inhabitants.
From these subjugated worlds, the Shi'ar have assembled a collection of powerful beings under their leadership, including the royal protectorate known as The Imperial Guard, and the more subversive and ruthless Death Commandos. While they have fought alongside earth-based heroes such as the X-Men on many occasions, the species aligned under Shi'ar rule are fiercely loyal to the throne and whoever sits upon it, leading to conflict and conquest when it has been usurped amidst multiple short-lived coups.
Kree
A technologically advanced race at a genetic dead-end, the Kree have spent thousands of years conquering neighboring and distant worlds and interbreeding amongst genetically similar populations in an attempt to restart their own stagnant evolution. This pursuit of biological expansion has brought them to earth on more than one occasion, directly resulting in the birth of Carol Danvers, whose half Kree DNA would eventually lead to her becoming the hero, Captain Marvel.
Led by their ruler, The Supreme Intelligence, an artificial being comprising the collected knowledge of all deceased members of their species, the Kree tend to think of themselves as a highly logical and emotion-eschewing empire. That identity is easily disproven by their millennia-long war with the Skrulls, a rivalry born from an early misunderstanding upon the species' first encounter.
Skrulls
One of the earliest alien species introduced in Marvel Comics, the Skrulls are a shapeshifting reptilian race who have warred with the Kree for millennia. Before their war with the Kree began, however, the Skrulls were a mostly peaceful species who developed faster than light interstellar travel early in their history, which they used to seek out other intelligent species to establish trade with. Landing on Hala, homeworld of the Kree, the Skrulls declared Hala's other dominant species, the plant-like Cotati, to be the more worthy representatives of the planet.
This perceived insult against them galvanized the Kree, eventually beginning their endless rivalry. As the war progressed and consumed their culture, the Skrulls devolved into a more aggressive species, conquering other worlds to expand their reach for strategic purposes. It was this expansion that eventually brought them to earth, where they have battled the human heroes on numerous occasions, once using their shape-changing abilities to infiltrate multiple heroic teams including the Avengers and Fantastic Four in a Secret Invasion.
Brood
A parasitic, insectoid alien species inspired by Ridley Scott's 1979 film, Alien, the Brood reproduce by implanting their eggs into other living beings, which then hatch, consuming the host and transforming it into a new Broodling. During this transformation, the new Brood gains the knowledge, memories, and abilities of the consumed host, resulting in many super-powered members of the Brood amongst their species.
When first encountered by the X-Men during an adventure in space, the Brood were able to capture and assimilate the entire team, save for Wolverine, whose healing factor rejected the alien gestating within his body. Since identifying earth's super-powered population as a potential resource, the Brood have attempted invasion on a handful of occasions, only to be thwarted by heroes such as Captain Marvel and the X-Men.
Technarchy/Phalanx
The Technarchy and Phalanx are technically two distinct species but are biologically intertwined. The Technarchy is a techno-organic species that infect living tissue with the "Transmode Virus," converting organic beings into a similar techno-organic form, known as a Phalanx. They then consume the energy of the Phalanx for nourishment, leaving nothing but a burnt-out husk.
When a being transformed into a Phalanx goes unconsumed, it retains the ability to infect others with the Transmode Virus, converting them into more Phalanx and absorbing them into their gestalt hive mind. Each Phalanx hive is driven by a biological imperative to summon the Technarchy to complete their purpose of being consumed. The Transmode Virus is highly contagious, making any conflict with the Phalanx or Technarchy inherently dangerous.
Celestials
Created from the sentient universe that existed before the current Marvel multiverse, the Celestials are vastly powerful cosmic beings who have studied and guided the evolutionary progression of life throughout the universe. Standing at thousands of feet tall, and made up of an amalgam of smaller, pseudo-independently intelligent organisms, each Celestial serves a unique purpose amongst its ranks. The overall goal of the species, however, remains a mystery.
Encountering and modifying various burgeoning civilizations throughout time and space, the Celestials machinations are responsible for the offshoot of humanity known as The Eternals, whom they periodically check in on to judge, along with the progression of humanity as a whole. Able to manipulate matter and energy on a massive scale, the Celestials are virtually unkillable, with the exception of a handful of ancient entities and weapons.
Beyonders
A near omnipotent alien species from outside the natural multiverse, the Beyonders have effortlessly killed cosmic beings such as the Celestials in various alternate realities. The first Beyonder encountered by the heroes of Marvel's earth sought to learn about humanity and its motivations, capturing a collection of heroes and villains and forcing them to battle in The Secret Wars.
Later Beyonders would deem multiversal humanity too dangerous itself to continue existing, and set about to destroy the multiverse, creating a blank slate from which they could start anew. With incredible reality-warping powers, the attempted destruction of the multiverse by this group of Beyonders eventually led to the merging of Marvel's Classic and Ultimate Universes into one singular reality.