Which do you think is the most threatening alien species to threaten us that you have fought back against? As per request I shall give some background on each answer: [b]Covenant[/b]: The covenant are made of a multiple species, united under the religious worship of the Forerunners and their belief that Forerunner ringworlds known as Halos will provide a path to salvation. the High Prophets declared humanity an affront to their gods, then the Covenant prosecute a lengthy genocidal campaign against the technologically inferior human race. [b]Chimera[/b]: an alien species hailing from an unknown star system. Originally, it was believed that gruesome Russian experiments resulted in the creation of the Chimera, due to their invasion starting out of that country. However, one of the antagonist alien bosses claimed that the Chimera were the original inhabitants of Earth and to be "the evolution of man". Whether or not this is true has yet to be seen but he could simply be stating that the Chimera are superior to the Human race. No one knows exactly the origins of the Chimera but we do know that they spread their own virus that infects people and turns them into more Chimera. More battles won = more soldiers. It is known that the Chimera invaded Earth seemingly for the sole purpose of eradicating humankind and claiming Earth for themselves. In Resistance , they terraform Earth itself and even open a wormhole in time and space. They bring humanity to the brink of extinction [b]Helghast[/b]: In the Alpha Centauri system there are 2 planets. Vekta and Helgan. Vekta was a lush garden world and Helghan was harsh brutal inhabitable wasteland. The Helghast are the original colonists of Vekta. The Helghan corporation purchased these planets (Vekta included) but Helghan's economic policies threatened to undermine the UCN's financial system. the UCN invaded Vekta and forced the Helghast off of Vekta after failing to impose financial sanctions. The planet's harsh environment forced the Helghast to adapt and mutate so much that they can no longer be considered human. They are stronger, faster and more resilient than their human cousins, and possess a burning hatred for humanity. Scolar Visari became the Autarch of the Helghast and forged them into what they are. He took the weak and helpless colonists on this forsaken planet and turned them into the merciless soldiers they are now thus creating the most powerful military force in the galaxy with their eyes dead set on Vekta with only one goal in mind. Revenge [b]Reapers:[/b] The Reapers are a highly-advanced machine race of synthetic-organic machines. The Reapers reside in dark space. A massive space between galaxies. There they are dormant for fifty thousand years at a time, before returning to the galaxy in a galaxy wide harvest of the alien races when they reach the peak of their technologic advancement. These giant machines are ancient. They may even date back to the dawn of time. Their true name is unknown. In the end, the Reapers spare little concern for whatever labels other races choose to call them, and merely claim that they have neither beginning nor end. The Reapers were thought to be a myth and were used in stories. The Reapers may be machines that are seemingly impossible to destroy but it doesn't end there. They have their own forces that do their fighting for them as well. They use a process called "indoctrination". It's basically "brainwashing" other organic life forms from the mere technology of the Reapers themselves. Indoctrinated people are basically people who are brainwashed by the Reapers into believing that they should fight FOR the Reapers instead of against them and that their whole civilization isn't worth fighting for and that they are delaying the inevitable by fighting them. These indoctrinated ones act as sleeper agents and act like normal people and then turn on their own. Results of indoctrination are husks, abominations, cannibals, banshees, and other such wretched creatures.