Symbiotes

The Symbiotes were scattered across the galaxy after their home planet was devoured by the world eater Galactus. For some reason Galactus could not siphon the life-force from the symbiotes themselves, however this event has left all the symbiotes psychologically traumatized. The race is now nomadic and live on various worlds throughout the cosmos, many who are related to the Venom symbiote live on Earth.

Symbiotes often have a give and take relationship with their hosts, but usually more take then give. When Spider-Man first was bonded to the Venom symbiote during the Secret Wars, it seemed too good to be true, he no longer needed web shooters or web-fluid, was stronger, and could morph the symbiote at will between street cloths and his costume, but the negatives quickly became apparent, Spidey became more violent, and was constantly exhausted, which he discovered was because the alien was taking his body out on patrol in his sleep. Spider-Man rejected the alien and it found it's way to Eddie Brock. Since then there have been numerous other symbiotes, most are relatives of the Venom symbiote who were born on Earth.

Biology

Symbiote biology is a bit conflicted between generations, but the seemingly undisputed facts (except in the case of Anti-Venom) is that they amplify the physical strength, agility, and healing of their hosts, are vulnerable to sonics and fire, reproduce asexually, and seemingly grow stronger with each generation. Symbiotes also have an amazing camouflage ability, and it seems they can absorb the abilities of previous hosts (having bonded with Spider-Man it gave later hosts wall-climbing abilities), they are empathic, and usually drawn towards negative emotions, and feed heavily on the chemical produced by the brain phenethylamine, the drain of phenethylamine is the very thing that leads the hosts of the symbiotes to become violent and mentally unstable, and is the largest drawback to being the host of one.