The Skyline Aliens are black cephalopod-like creatures with the ability to transfix and utilize the brains of different species to power themselves.
History
The Skyline aliens invading Earth.
Nothing is known about the origin of this alien species. What is known is that they arrived on Earth and began harvesting humans globally, using their brains to power themselves up. They managed to "harvest" most of the human population, but nothing is known about the aftermath. The surviving humans were filled with rage and fury.
Varieties
Drones
Drone.
Drones are squid-octopus like aliens with multiple eyes and glowing lights on their tentacles that they use to bring other species into a trance-like state. Drones have the ability to steal the brain and spinal cords of their prey. Drones have slimy black skin. Drone's are capable of surviving without a Brain but its only for a few minutes; in that minutes a Drone's tentacle or all of the Drone approaches the most close target who have a suitable brain. Drones can be destroyed...temporarily...
Hydras
The Hydras are black flying aliens that function as dropships. They patrol regions being attacked and release drones to find prey. They have wings and one big blue eye. They are used as "suicide bombers" when combating enemy aircraft. It is surprising that they do not have any projectile-based weaponry, considering how advanced they are supposed to be. They are also seen hunting down hiding humans.
Tankers
Tanker
How a Tanker 'catches' its 'Prey'
Tankers are giant crab-frog like creatures with multiple eyes and 8 brains for "power". Instead of hypnotizing prey with blue lights, they have clingy tentacles which they use to hunt. They walk/run on four legs and are olive green in color.
They are incredibly strong and almost impossible to destroy but their skin is flammable and this capable of slowing them down. They're mostly 'gathering' the alive prey much as possible and then they take them into a mothership for the use of their brains. Its also logical to say they're the main 'gathering' forces of the Skyline Alien Species.
It has been hypothesized by some that the tankers are the result of the aliens adapting the human's physical appearance and fusing this functionality into their own forces. It has been questioned why the tankers only appear after the initial abduction.
It also can be hypothesized that these aliens' true form is really unknown, as they could have simply been taking the appearances of other alien civilisations (like humans) which they captured and subdued. It is illogical to assume conveniently that the aliens already had a human like form when they first reached earth.
Workers
The purpose of this variation of Skyline aliens is unclear. They could be workers, guards or soldiers. They are quadrupeds that resemble smaller tankers. They have melon-shaped heads with murones. Every individual has multiple blue eyes and slimy, dark brown skin. Like the drones, they need a stolen brain in order to function. A Worker is seen inside a ripped-open drone towards the end of the movie, suggesting they may be the actual true species, who simply "pilot" the other lifeforms. These could be the adaptation of humans into their species, at least, similar in form to humans.
Abilities and anatomy
Most of these aliens have the ability to hypnotize a host and feed/power up by "brains". They have a blackish-purple blood (the purple hints at haemoglobin). The aliens suck prey into their body through holes or mouth-like orifices. Only the Drones have the 'power' to transfix in this way. This is unlikely to be an evolutionary trait, due to its sophistication and seemingly impossible effect. It could only be technology that they developed. They are able to regenerate lost tentacles with great speed.
The blue light they generate to hypnotize can also enhance the creatures that escape the initial hypnosis. An example was Jarrod (having been saved earlier from the alien's hypnosis) showing increased strength and aggression, with his eyes glowing blue and the skin/veins around his eyes darkening - just the same as what happened when he was first hypnotized. His increased strength was shown first when he lifted and pinned Oliver against the wall as he vowed to protect his family. It was shown again when Jarrod and Elaine were on the roof before been attacked by a drone. When the alien attacked Elaine and tried to take her brain, an injured but strengthened Jarrod knocked the alien off her before physically attacking it by hitting it with a breeze block and then using his fists. Finally, he ripped the drone open before tearing out the human brains it had acquired.
Technology
A mothership of the Skyline aliens
The technology of the aliens seems to be a fusion of metals and organic components, suggesting an extremely advanced biomechanically driven species. According to Colin Strause, even the motherships are creatures of some kind with their own DNA. Some of the machines/creatures seem to be able to rebuild themselves even after a nuclear strike, though these might be destroyed if the explosion is in the interior.
This is a list of machines:
The motherships are made with a metal hull and organic substances on the inside, a tractor beam, and the device that enables the aliens to hypnotize. They look saucer-like, but with spirals and arches sticking out.
Their abduction devices are tractor beams, accompanied by a bright blue light, that can suck Humans into the air, and possibly descend from their ships.
The 'alien sorter' is a thin robot with a blue light on the front and a 3-pronged claw, razor blade, and a tube that sucks up the brains into a room that attaches it to the aliens.